Voice over IP

Telephone Trunk Lines: What is a SIP Trunk?

Like how you can use an IP Phone/ ATA to make inexpensive long distance calls from your home using a VOIP Service, SIP Trunks enable the users in an organization to do the same. To use a SIP Trunk, you need an Internet Connection, an IP PBX/ Analog-Mixed PBX, IP Phones / Analog Phones, ITSP (SIP Trunk service provider) and users who want to make outgoing calls! Let us look at SIP Trunks a little more deeply, in this article.

What are Telephone Trunk lines?

Telephone trunk lines are the ones that enable users to make outgoing calls / receive incoming calls in an organization. We are talking about land-line calls, not cell phones!

So, in an organization there might be many users with either IP Phones or Analog Phones and they connect to the IP PBX or Analog-Mixed PBX. This is on the user end.

On the other side, you can get some trunk lines (Analog Trunks, ISDN Lines/ PRI-E1-T1 Lines, etc) from a Public Telephone Exchange or Telecom Service Provider (PSTN) and terminate them to the PBX as well.

So, when a user makes an outgoing call with his analog phone/ IP phone, the call reaches the PBX and the PBX will connect the call to any of the free Trunk Lines. Similarly, when there is an incoming call, it lands up on the PBX and the PBX routes it to the appropriate extension.

Click here to read the difference between Analog Trunks, PRI/E1/T1 Lines and Cellular Gateways. They are all Telephone Trunk Lines and they are also called PSTN Trunks (Public Switched Telephone Network).

What is a SIP Trunk?

One of the biggest advantage of VOIP / IP Telephony is its ability to enable users to make inexpensive long-distance calls as these calls are transported on the Internet and hence it can by-pass the normal toll charges applicable for analog & digital trunks for inter-state / inter-country calling.

So, a SIP Trunk is a virtual IP based trunk line that uses the Internet to make calls / receive calls to land-line numbers, cell phone numbers and other VOIP numbers. The SIP Trunk is terminated on the PBX (both IP PBX and supported Analog-Mixed PBX models) and the user’s call can be sent over the Internet.

There is a SIP Trunk provider called ITSP – Internet Telephony Service Provider who has connections with other VOIP Service providers/ PSTN Service providers to enable VOIP phones to call PSTN phones and vice versa.

So, when an international call is made by the user, the PBX connects this call to the SIP Trunk, it reaches the ITSP over the SIP Trunk (via the Internet), it is carried to the destination country/city over the Internet / packet switched network by the ITSP and then transferred to the local PSTN exchange over there to reach the destination number.

What are Private SIP Trunks?

SIP Trunks also refer to the Interconnectivity of two private IP PBX’s located at different places over the Internet / IP Network. This creates a (Virtual) Private Network between the two of them so that users at one location can call/receive calls from users in the other location without incurring additional call charges.

For example, if there are two branches of the same company in City A and City B, the PBX in City A can be Trunked over the Internet/ Leased Lines/ IP Networks (using SIP Trunks) with the PBX in the City B. The calls between the users of these two locations do not incur additional cost as these are internal calls carried over existing network.

Salient Features of SIP Trunks (Public/ ITSP):

  • SIP Trunks connect to the Enterprise PBX over the Internet through an ITSP – Internet Telephony Service Provider/ Business VOIP Service provider. They enable VOIP users to call PSTN numbers and vice-versa.
  • SIP Trunks are virtual circuits that use the existing Internet connection (Internet Leased Lines) for physical transport of the voice packets.
  • Using SIP Trunks, users can make calls to local PSTN numbers, long distance land line numbers, cell phone numbers and other VOIP numbers.
  • SIP Trunks come with their own telephone numbers, just like PSTN phone numbers which can be reached from anywhere. It is possible to get a local phone number with a local area code (and) toll free numbers.
  • For each IP call, it is recommended to provision at least 80 kbps of upload / download Internet bandwidth. So, symmetric Internet connection like Internet Leased Lines are always better than business broadband.
  • ITSP’s offer calling plans for SIP Trunks like normal telecom/ PSTN service providers. There is either a fixed rental for each line every month  offering some free calls and charging for additional calls (or) there are unlimited calling plans for each line with a flat monthly rental.
  • SIP Trunks can be obtained quickly (usually within a single business day) and individual line increments/ decrements are possible.
  • SIP Trunks can provide Local Number Portability, Direct Inward Dialing, Enhanced 911 Services, and other services provided by E1/T1 trunks.
  • SIP Trunks can co-exist along with Telecom/PSTN trunks in a PBX and the PBX can choose the best trunk for each call, based on certain pre-programmed parameters.


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