Configuring Lync 2010 Server to Work with Level 3 SIP Trunking Services

Interested in using Level 3 SIP trunking services with your Lync 2010 deployment? Connecting SIP trunking services can be a challenge. It’s difficult to determine what service providers support and don’t support. That’s why the good folks on the Microsoft Lync Online Dedicated Deployment team prepared this configuration guide--to assist you with your next SIP trunking installation using Level 3 services.

Author: Larry Felt, Microsoft Senior Service Engineer

Technical Reviewer: Rob Pittfield, Microsoft Senior Service Engineer

Editor: Susan S. Bradley

Publication date: April 10, 2013

Product version: Lync 2010 On-Premise, Lync Online Dedicated

When I was preparing to deploy Level 3 SIP trunking services for one of our Lync Online Dedicated customers, I searched the web for examples of “known good” Lync configurations that worked with Level 3 SIP trunking services and had a difficult time finding solid information. Working with other service providers, I know that there are always configuration issues that you need to address during the initial deployment and testing phase. This article will help you get SIP trunking services deployed and tested quickly, by providing the configuration details required for Lync 2010 Server.

Level 3 SIP Trunking Configuration

Ordering Services

Level 3 has been qualified in the Unified Communications Open Interoperability Program (UCOIP) for Lync On Premise deployments, as well as for the Lync Online Dedicated Service.

What does this mean to you? It means that Level 3 tested its commercial SIP Trunking Services with Lync 2010 and with the Lync Online Dedicated Service offering and passed all required interoperability tests. For information on ordering SIP Trunking Services from Level 3, see Level 3 SIP Trunking with Nomadic E-911 for Microsoft Lync.

NOTE Level 3 natively supports SIP over Transmission Control Protocol (TCP) and does not require any additional on premise equipment, such as SIP PSTN Gateway or session border controller (SBC) when connecting to Lync Server.

Connecting the SIP Trunk

This document does not cover connecting the SIP trunk to your Lync deployment. I assume you have the SIP trunk connected and ready for Lync configuring and testing. For connectivity options available for Lync Server, see How Do I Implement SIP Trunking?

Figure 1. SIP trunking connectivity options

Configuring Lync Server

The following section provides the configuration methods and details required to configure Level 3 SIP trunking with Lync 2010 using TCP, not TLS for SIP Signaling. At this time, Level 3 does not support TLS/SRTP in its commercial SIP trunking offering.

Configuring the Topology

Using Lync Topology Builder, you must create the public switched telephone network (PSTN) Gateway object (Signaling IP for Level 3 SBC) and associate it with a Mediation Server Pool, as follows.

To create the PSTN Gateway object and associate it with a Mediation Server Pool

1. Launch Topology Builder.

  1. Enter Gateway IP Address (provided by Level 3).
  2. Enter Gateway listening port (5060).
  3. Make sure TCP is checked.


Figure 2. Edit Mediation Server Pool

2. In the Mediation Server PSTN gateway dialog box, edit Mediation Server pool properties.

  1. Check Enable TCP port.
  2. Change TCP port to 5060 (default is 5068).
  3. To associate the new PSTN Gateway with the Mediation Server pool, click Add.


Figure 3. Associate PSTN Gateway

Configuring the Trunk

You must create a new trunk to be able to set the custom configuration needed to interoperate with the Level 3 SIP Trunking Service. A Lync administrator can configure most of the trunk settings using the Lync control panel, but some of the configuration settings needed are only available by using the Lync Management Shell. Here are the details for configuring the trunk for both methods.

Sample Trunk Configuration for Level 3


OutboundTranslationRulesList : {Description=Remove + from N11 calls;Pattern=^\+([4,5,7,9]\d{2})$;Translation=$1;Name=Remove + from N11 calls} 1

SipResponseCodeTranslationRulesList : {}

Description :

ConcentratedTopology : True

EnableBypass : False

EnableMobileTrunkSupport : False

EnableReferSupport 2 : False

EnableSessionTimer 3 : True

EnableSignalBoost : False

MaxEarlyDialogs : 20

RemovePlusFromUri : False

RTCPActiveCalls 3 : False

RTCPCallsOnHold 3 : False

SRTPMode : NotSupported

EnablePIDFLOSupport : False

Configuration settings required:

  • Outbound Translation Rule for N11 calls: 1
  • Disable SIP Refer configuration: 2
  • Disable RTCP configuration: 3

To configure the trunk using Lync Control Panel

1. Open Lync Control Panel, click Voice Routing, click Trunk Configuration, and then click New Pool Trunk.

2. Choose the PSTN Gateway object you previously configured in the topology.


Figure 4. Choose PSTN Gateway object

3. For Encryption support level, select Not Supported.

4. Uncheck Enable refer support. (This will disable SIP refer method as required by Level 32)



Figure 5. New trunk configuration

5. Create the outbound translation rule.

  1. From associated translation rules, select New.
  2. In the Name text box, type Remove + from N11 calls.
  3. In the Description text box, type Remove + from N11 calls.
  4. In the Starting digits text box, type +[4,5,7,9] (This covers 411,511,711 and 911 calls.)
  5. In the Length text box, type exactly 4.
  6. In the Digits to remove text box, type 1.
  7. To commit the changes, click OK.

This will configure the outbound translation rule required for N11 calls by Level 31.


Figure 6. New translation rule

Level 3 has a requirement for N11 calls, such as 411, 511, 711, and 911, where they do not want to see the leading “+” in the “To:” Header on the SIP Invite. They are fine with the heading “+” for all other call types, such as local, long distance, or international calls. An outbound translation rule has to be created and applied to the trunks in the deployment in order for Level 3 to resolve this configuration issue.1 You must disable the RTCP parameters using the Lync Management Shell. They are not available in the Lync Control Panel.

6. Open Lync Server Management Shell, as follows:

Set-CsTrunkConfiguration -Identity PSTNGateway: -EnableSessionTimer $True -RTCPActiveCalls $False -RTCPCallsOnHold $False

Level 3 does not support SIP Refer for Call Transfer or Real Time Control Protocol (RTCP). A symptom of not supporting RTCP is that calls will disconnect after a period of time ( > 30 sec) on hold or during a Call Park. Lync 2010 has trunk-specific configuration to disable SIP Refer and also RTCP. 3

7.  Configure the Trunk, using the Management Shell only, as follows:

New-CsTrunkConfiguration -Identity PSTNGateway: -EnableSessionTimer $True -RTCPActiveCalls $False -RTCPCallsOnHold $False –EnableReferSupport $False –SRTPMode NotSupported

New-CsOutboundTranslationRule -Parent PSTNGateway: -Name "Remove + from N11 calls" -Description "Remove + from N11 calls" -Pattern '^\+([4,5,7,9]\d{2})$' -Translation '$1'

Congratulations! The trunk is now added to the Lync topology and configured to support Level 3 SIP trunking services. Now, you must add this PSTN Gateway to your Lync call routes.

Configuring Lync Server to work with SIP Trunking Services can sometimes be a challenge, especially determining what configuration settings are required to interoperate with each service provider. The purpose of this configuration note is to provide the reader with a “known good” Lync configuration for Level 3 SIP Trunking Services.

Additional RTCP Information

Calls on hold or on call park drop after 30 seconds

The mediation server is configured to use RTCP. After the gateway receives a=sendonly or a=inactive in the SDP, it doesn't send any media traffic (no RTCP). The server ends the connection with RTCP timer is not disabled and Mediation Server encountered a gateway media stream timeout.

To resolve, set the mediation server setting with this switch to disable RTCPActiveCalls and RTCPCallsOnHold: Set-CsTrunkConfiguration -RTCPActiveCalls $false -RTCPCallsOnHold $false -EnableSessionTimer $True


This parameter determines whether RTCP packets are sent from the PSTN gateway, IP-PBX, or SBC at the service provider for active calls. An active call in this context is a call where media is allowed to flow in at least one direction. If RTCPActiveCalls is set to True, the Mediation Server or Lync Server client can terminate a call if it does not receive RTCP packets for a period exceeding 30 seconds. Note that disabling the checks for received RTCP media for active calls in Lync Server elements removes an important safeguard for detecting a dropped peer and should be done only if necessary. The default is True.


This parameter determines whether RTCP packets continue to be sent across the trunk for calls that have been placed on hold and no media packets are expected to flow in either direction. If Music on Hold is enabled at either the Lync Server client or the trunk, the call will be considered to be active and this property will be ignored. In these circumstances use the RTCPActiveCalls parameter. Note that disabling the checks for received RTCP media for active calls in Lync Server elements removes an important safeguard for detecting a dropped peer and should be done only if necessary. The default is True.


This parameter specifies whether the session timer is enabled. Session timers are used to determine whether a particular session is still active. Note that even if this parameter is set to False, session timers can be applicable if the remote connection has session timer enabled. In such a case, the Mediation Server will reply to session timer probes from the remote entity. The default is False.

Additional Information

Lync Server Resources

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Keywords: SIP Trunking, Level 3, PSTN

Comments (3)
  1. John Hearty says:

    I don't know Lync config details, but the below may not always apply; Level 3 supports Nomadic 911 if the customer wants it.

    EnablePIDFLOSupport : False

    John Hearty

    Level 3 Voice Engineering

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