Overview and Readiness for configuring SIP trunk in Lync 2010/2013

What is SIP trunk?

SIP trunk is  a virtual  connection between Lync server(mediation server) in any organization  and the ITSP.SIP trunk is used to make a direct connection from your organization to ITSP(Internet Telephony Service Provider).

It helps us to extend any organization VOIP (voice over internet protocol) functionality beyond the firewall without the need of ip-PSTN or even a PBX.

In a simple term SIP trunk can be utilized to carry VOIP calls in following scenarios

1)  PC to phone Communications.

2)  Phone to PC Communications.

3)  PC to PC    Communications.

In addition to VOIP calls  SIP trunk can carry instant messages, multimedia conferences, user presence information, Enhanced 9-1-1 (E9-1-1) emergency calls, and other SIP-based, real-time communications services.

In some scenarios we can choose the SIP trunk to connect to the branch site through ITSP.


Advantages of SIP trunk

By using SIP trunk we can typically reduce the cost in making a long distance calls.

We do not need to have a PSTN or even a PBX.

We can consolidate our organization connections and PSTN in a single central site.

An enterprise user can make local calls i.e., within the organization and a long distance call to a  E.164-compliant number.


What is  E.164-compliant number ?

International public telecommunications numbering plan. The E.164 plan was originally developed by the International Telecommunication Union (ITU).

E.164 is an international numbering plan for public telephone systems in which each assigned number contains a country code (CC), a national destination code (NDC), and a subscriber number (SN). There can be up to 15 digits in an E.164 number. With E.164, each address is unique worldwide and it has its own number. With this 15 digits combination for each user there can be 100 trillion numbers generated all around the world. So there is no possibility in future that  we might run out of option in assigning E.164-compliant number for any user.

SIP trunk can be established in 2 ways

Centralised SIP trunk

Is a method in which all the branch site  connections come to a central site and then the connections are routed to the ITSP. This type of connection is Cost effective.

Distributed SIP trunk

Is a method where all the sip trunk are established of its own in each and every site to their own ITSP.

Reason for having Distributed SIP Trunk

Company sites can be in different countries

In order to provide Site resilience


How  do we implement SIP trunk ?

1) Order a SIP trunk subscription connection from any one of SIP trunk service providers.

2) Configure the external firewall to allow communication between SIP Trunk provider and your Lync Server with the Mediation Server role.

3) Make the necessary changes and publish the Lync Topology.

4) Create a dial plan and configure users for using enterprise voice.


SIP trunk service providers can give 2 types of connections

1) TCP connection

For this type of connection we need to have port 5060 open in the firewall for SIP trunk provider. For this connection there is no certificate required. This type of connection uses NTLM type of authentication since no certificate is required.

2) TLS  Connection

For this type of connection we need to have port 5067 open in the firewall for SIP trunk provider and then we need to get a certificate from the SIP trunk provider. This type of connection uses TLS-DSK since it uses a certificate for the connection.

We need to exchange few information between the SIP trunk service provider and our organization to configure the endpoints on both side for this type of connection.This configuration basically depends on what type of connection we are using for this SIP trunk and varies according to  different scenarios.

For MPLS and dedicated type of connection ,give the router public IP address to  ITSP . Ensure that ITSP’s SBC or gateway is able to resolve this address.

Also give the FQDN of the Mediation server.

For VPN connection, give the VPN server IP address to the ITSP

Note :

We need to get the ITSP gateway.i.e, their IP address and the FQDN .We need to specify these values in the mediation pool of the mediation server.


We need to have a Mediation server with 2 interfaces Basically 2 NIC’s.

Internal interface connects to the front end server for internal connection.

External interface – connects to ITSP edge component.(the external edge component of ITSP can be a router,SBC or an gateway)

The mediation server can either be collocated along with the front end server or it can be a standalone server according to the requirement. It’s always better to use the Lync server  planning tool to evaluate the front end server pool that whether it can handle the load. Else we can deploy the mediation server separately.


If you are connecting to a SBC(session Border Controller) of a ITSP a dedicated Mediation server is required.

We need a connection to establish a SIP trunk from mediation server external interface to the ITSP external edge for any organization. Below are the supported connection types

1) Least fibre  Optic connection or T1 line

This one has the highest call carrying capacity, most secure and reliable one. Most expensive. Suitable for organization who prefer good quality of call and security. No VPN is required.

2) MPLS connection

Multi Protocol Label Switch is a dedicated line shared with other IP traffic and the bandwidth network is shared with other subscribers. No VPN is required. Drawback is that it can have excess of IP traffic,can interfere with VOIP unless VOIP is given as highest

3) Internet connection

Least expensive. VPN connection is required but less secure and low  quality.


Bandwidth requirements

Bandwidth planning is very important before making SIP trunk connection. We need to calculate according to the number of calls. That can be calculated with the below formulae

SIP Trunk Peak Bandwidth = Max Simultaneous Calls x (64 kbps + header size)


Codec Support

Lync Server 2013 supports only the following codec’s:

G.711 a-law (used primarily outside North America)

G.711 µ-law (used in North America)


Internet Telephony Service Provider

For ITSP you need to use only Microsoft certified ITSP.

How you implement the service provider side of a SIP trunk connection varies from one ITSP to another. For deployment information, contact your service provider. For a list of certified SIP trunk service providers, see Microsoft Unified Communications Open Interoperability Program website. For details about Microsoft certified SIP trunk providers, contact your Microsoft representative.

References :-




Cheers 🙂

Sathish Veerapandian

Technology Evangelist

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