What is a PRI Line, what are the advantages and limitations of PRI circuits

This article explains what a PRI line is, what are the benefits of having PRI lines for the telephony requirements of an organization, and the dis-advantages of a PRI line.


PRI Line Connectivity and Architecture Diagram

What is a PRI line?

  • There is only one line physically terminating on the customer PBX but still a PRI line can receive/send 30 calls simultaneously! A PRI line is end to end digital circuit.
  • A PRI (Primary Rate Interface) line is a form of ISDN (Integrated Services Digital Network) line which is a telecommunication standard that enables traditional phone lines to carry voice, data and video traffic, among others.
  • A PRI circuit consists of two pairs of copper lines terminating on a modem from a service provider premises to the customer premises. It uses multiplexing/de-multiplexing techniques to carry more than one channel in a single circuit. There are two common forms of PRI lines – E1 (which carry 30 channels in the two pairs of copper lines, common in Europe, India) and T1 (which carry 23/24 channels in the two pairs of copper lines, common in United States).
  • Each channel in a PRI line provides 64 Kbps for data transmission.
  • A PRI line can connect to both Analog/Mixed EPABX systems and also the newer IP PBX systems. A PRI Card / Interface might be required to terminate the PRI circuit on the PBX.
  • A PRI line can also be used to connect two PBX systems thereby providing 30 channels between them for interoperability.

Advantages of PRI Lines:

If thirty separate analog trunks are taken instead of one PRI line,

  • The cost of terminating all the thirty analog trunk lines becomes higher than terminating one PRI line.
  • There would be thirty rentals to be paid instead of one consolidated lower rental for a PRI line.
  • Some analog trunks might be used more (uneven distribution of calls) and some lines may not have even crossed the free calls limit.
  • Terminating 30 analog trunks in a PBX also requires more free slots/cards than the one slot usually occupied by one or even two PRI trunk cards.
  1. Direct Inward Dialing: For each PRI line, the service provider would provide more around 100-500 numbers which can be used by outsiders to call the extension directly, instead of having to go through the PBX Auto-attendant.
  2. Caller ID: Since all the extensions have their own number, this unique number will be displayed in the phones that they are calling to. Some call centre applications are based on the unique caller ID number for differentiation of services.
  3. It is possible to offer both voice and data in the PRI line. Some service providers have dynamic offerings where data is transmitted in all the channels that are free (not occupied by voice) at that given point of time.
  4. Call hunting (Where the call lands in any channel that is free, instead of the called number specifically – For example, if there is one board number but a number of people are calling in at the same time and still a channel is allocated to them .With analog lines, if one number is busy, they need to call in another number manually) is possible by default with a PRI connection, but for the analog trunks this facility needs to be extended by the service provider and enabled on the PBX, involving additional cost at times.
  5. PRI lines can be used for voice connectivity, data connectivity, video conferencing, faxing, and all the above can be done simultaneously too (on different channels).
  6. PRI lines are end-to-end digital lines and hence the clarity is much better than analog trunk lines.
  7. Since they are digital lines, PRI lines are more reliable and trouble shooting is also easier with them. They are mostly on a fiber core ring and hence there is some redundancy.
  8. It is harder to tap into digital lines and listen to the conversations.
  9. There are flexible billing options available with most of the PRI service providers. The billing can be centralized or distributed (department wise, etc).
  10. PRI lines take lesser time to establish calls then analog trunk lines.
  11. Some service providers offer flexible plans where instead of the full 30 channels, they provide and charge for only 20 channels etc. This makes PRI lines more economical for smaller companies.

Dis-advantages of PRI lines:

  1. A PRI line is economical only if the minimum rental charged by the service provider for a PRI line is more than the average value of calls with analog trunk lines every month in an organization. Otherwise, the usage may not even cross the free call value provided by the service provider for a PRI line.
  2. A PRI line is not so economical for long distance/ international calling. An ITSP or SIP trunk service provider who takes the calls over the internet might charge much lesser for international long distance calls.
  3. Inter branch communication between the branches is not free of cost with PRI lines (Some PRI service providers provide this facility, but all your branches may need to have PRI lines from the same service provider and there also might be a minimum revenue commitment for the same). With VOIP systems, inter-branch communication can be done over internet/ leased lines hence reducing the cost drastically.
  4. The cost of a single PRI card to connect to your EPABX/ IP PBX is still very high. Most of these cards are proprietary, meaning you can buy them only from your EPABX vendor.

 Related Article: Difference between Analog Trunk Lines, PRI/E1/T1 Lines & GSM Gateway.

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        • Mahendra

          Hi Rajesh,
          Need Some expert guidance on usage of PRI line with DID numbers for STD numbers for economical purpose , please provide your number Thanks Mahendra

      • Fred

        If you have a rival you want to destroy, get him a BSNL PRI service. They don’t care if your business operates or not. Can you believe I have hired an employee just to get things sorted out with BSNL…!!! GO TO AIRTEL< VODAFONE OR FOR THAT MATTER EVEN TATA, but stay away from BSNL

        • BASHA

          Might be an agent for private operators. Voicing unestablished things about BSNL. Perhaps BSNL is the best service provider by any means in India. It is better late than never. You may feel it very late.

        • Anonymous

          I agree with you. We also face many problems with BSNL PRI. Worst after sales service. Very irresponsible staff, they dont care for their customers, all they want is their Salary at the end of the month.

        • Anonymous

          The sceneries changed . The BSNL staff’s attitude are changing. The cheapest and best network. Dont forget that The basic foundation for telecom service is laid by BSNL only not AIRTEL, Relaiance etc

  • Sid

    What is the reason for drop in revenues in per line of PRI’s these days? There seems to be fall in the revenue generated according to recent analysis

  • jay

    Dear All

    thare are PRI teasters availabeinmarket with which you can check your pri line…

    Reliance’s pri line isbest as i expereinced….

  • puneet dudi

    I just want to do Infrastructural setup for bulk audio calling/SMS having the capacity 50,000 calls per day. For that I need 5 PRI lines. Do any one have any idea, who can provide me the best Software & Hardware solution for this.

  • Musleh

    Hi Sir,
    I need to install a fax solution using PRI/T1 line to divert (logically from Service provider side) all land line fax number to that PRI/T1 line & connect this PRI/T1 line to server by adapter/card to Control those lines by windows fax service.
    I need your kind consultation for the below questions:
    1- Is it possible to use this in any fax server solution that integrated with Windows Fax Service?
    2- Can Windows Fax Service see all fax numbers separately to receive Faxes for each line?
    3- Finally, which HW Adapter/Card solution to link PRI/T1 line to server that’s suitable and recommended for this project?

    Thanks for your time.

  • Vignesh Muthaiah

    Hi i’m Vignesh currently working in desktop Support Engg and Completed CCNA, now i got job in TTSL ( Tata )is network engg but Field work, like PRI ,Voice & Data ,Cisco Routers config ,now please suggest me ,how about future of this job & salary growth in future , this is good field, please

  • Robinson

    Can I Connect One PRI Card on a PABX Machine directly to another PRI card on another PABX Machine without using a modem. The distance between the two PABX Machines is 300M.
    Thank you

    • Rajesh K

      You can do PRI Trunking *using the appropriate cards in both* between two PBX, if the respective manufacturers support it. Check with the PBX manufacturer.

  • John

    actually i am not voice Eng can any one help me out with easy and simple explanation so i can understand properly .. actually i am work on design and implementation and our design we use PRI for voice. that i know only so how to figure out how PRI work and how its take action in network environments

  • Sukhen Das

    I m working as dialer manager in a company which name is serco bpo and basically i used to dialed for ROb and kolkata circle and i have 60 chanells for both the circle to dialed through genesys dialer,but i have a issue that i m unable to control abndoned %, my total manpower is 85 if i will dialed 1:1.5 ratio still the issue is happening and if i will dialed against 1:1 ratio then my process avail% is getting high,,
    please help me out zbt this serious concern and im really worried abt it.

  • JR

    Question on a PRI with an existing DID can I insert or Drop a Whole Number not related with the DID range?

  • Tiao

    Please can you write an article on both SIP and SIP-I. Still can’t get a full grip on the two protocols. Thank you.

  • raj

    i have t he requirement as follows:
    10 B channel PRI
    4 Analog extensions
    10 Digital Extensions
    40 SIP licenses .
    can you let me know how many number of PRI d0 need.and explain why you selected the particular number of PRI

    • Rajesh K

      You can replace the 10 B channel BRI, with one PRI line. Of course, you also need to buy the appropriate PRI interface card from your PBX vendor. One PRI can carry 30 channels, but one BRI can carry only 2 channels of voice.

  • Madhu Kumar

    Hi Rajesh

    I need to use our existing PRI line along with Voice for DATA communication can you guide me how and what equipment are needed to do this Configuration
    WE have BSNL PRI and we have Panasonic EPBX

  • Naseeruddin

    Hi rajesh

    we have 13 branches PRI and MPLS connections with same service provider but we are facing lot of fiber cut issues every day we have 3 to 4 units fiber cuts. is there any way that we can use PRI lines in RF (wifi) connectivity. we are planing to take backup line in RF for MPLS same like can we take PRI lines.

  • Amit Bajaj

    Dear Rajesh,

    Thanks for Excellent Article. I read this and some subsequent pages/articles well. I had lot of confusions which got vanished.
    Would be grateful if you could write something about Direct Inward Dialing.


    Hi! I m going to buy a PRI line for our college. But we have 5 BSNL land lines (Analog), which we want, bcoz these nos are published all over. Can I connect these lines(analog) & PRI lines? My requirement is that: my line should not be engaged at all times, it should be free if these line are joined to EPABX. If joined I should get 30 PRI + 5 land line nos= Total 35 incomig channels.
    Is it possible ? Plz guide.
    thanks !

    • Rajesh K

      You can connect your existing 5 landlines and 1 new PRI line to a single PBX. But, the PBX model should support it (check it with the PBX manufacturer). If it doesn’t, you’ll have to buy a new PBX.

      When you buy a PRI, you get 30 channels – so, you’ll not need the 5 landlines (I guess). Check if your telephone service provider will allow you to retain your existing phone number if you surrender the landlines and upgrade to a PRI. They might.

  • Lanre

    Assuming a PRI line has 10 available channels with ten DID lines that can be called on redirecting using the IPBX, how are the incoming/outgoing calls made?

    Is it that channel 1 is dedicated for the first number, channel 2 for the second number and so on or all DID numbers can pass through any channel at any time?


  • Hiren Raval

    I want to know about this PRI Line.As we want a new PRI line in our office.So if anyone please help me.And can provide some contact with good company who is providing the PRI line.

    • jhunnu

      No, two SP will not give you same number logically, asa you take one DID number from one provider that number is gone and other provider can’t assign the number, better keep same provider they can

  • Himanshu Pathak


    I would like to open a small home based call center for local clients.
    Initial we would like to start with 4-5 Agents. Approximately call would be 100-200 Calls / Day.
    Can you please help me to figure out what exact resource I required. And also the cost of PRI Line (License cost if any).


    • jhunnu

      Don’t go for PRI line will be expensive, go for 2 BRI line and later on if business grow upgrade to PRI. Can also consider VOIP much cheaper

  • Amber

    We are looking for a solution of voice blasting with PRI ISDN lines. We have the server and PRI lines installed at our company. Now we are looking for a application with which we can make voice broadcasting with IVR (Press button responses). Please suggest me with something since Asterisk is not understandable

    • Surya panda

      No bcoz that is possible on national calls..pri not success in long distance or international calls…

  • Suresh

    Hi every one. We have 15 branches of our company in Bangalore and we are using individual pri for every branch. As the cost for the PRI rent is very high is there any other solution where the service provider can provide us 1 PRI line and connect it to individual EPBAX available in all the branches.

    This will help us in saving a huge amount.

    Please suggest

  • Vinod Kumar

    Hi everybody I am working with government department. I am planning to take one PLI (30 channel) line for our department. I need to know if this PLI line dead then our whole telephone network (EPABX) will dead. If this is the true then what is the solution and any other suggestion regarding PLI line implementation.

  • V K Shakyawar

    Hi everybody I am working with government department. I am planning to take one PLI (30 channel) line for our department. I need to know if this PLI line dead then our whole telephone network (EPABX) will dead. If this is the true then what is the solution and any other suggestion regarding PLI line implementation.

    • Yogesh

      If you have single PRI line on PBX and if it’s goes down then ur complete business will impact. to reduce your impact then you can use two PRI from two different service provider and assign them as circular mode. Once any one goes down then call will automatically route through another PRI line. One more better solution is there that is GSM gateway on which you can use 30 sim cards which can be different services provider. Which will not impact operation.