I recently worked an issue where the default setting of “Enable refer support” caused a routing failure with an Enterprise Voice scenario. The customer is configured with a direct SIP trunk from their ISP that’s pointed to their Lync Mediation role. In this scenario, an inbound call from an external PSTN user hits the Exchange Unified Messaging 2010 (EUM) Auto Attendant (AA). The EUM AA was configured with a Lync Dial Plan. When the AA tried to connect the inbound caller to the Lync enabled internal user by the SIP REFER method, it failed to connect.
SIP Stack tracing from the Mediation Server logged the following SIP error:
SIP /2.0 400 Refer-To: user name not found
When reviewing a network capture from the Lync client, the VoIP call graph analysis showed the following:
|30.916 | REFER | |SIP Request
| |(5060) <------------------ (63788) |
|30.964 | 400 Refer-To: user name not found |SIP Status
| |(5060) ------------------> (63788) |
As stated before, the “Enable refer support” switch is enabled by default.
If you have deployed Lync with Enterprise Voice and utilize a “supported” gateway, SIP REFER works as expected. However, you are not using a “supported” gateway or have a SIP trunk being provided by a ISP that is not using a “supported” gateway or a gateway that does not support SIP REFER, you will need to disable the default setting for SIP REFER support. It is recommended that you verify whether or not the SIP trunk provider supports SIP REFER.