Lync Music on Hold with Media Gateways

With the CU7 release Microsoft has added Music on Hold abilities to the Lync Phone Edition clients. Many organizations are starting to turn this feature on now, but it's possible that external PSTN callers still might not not hear any music on hold being played. If you're seeing (or not hearing) this behavior, there may be a gateway setting that's not passing the music on hold to PSTN callers.

On an AudioCodes MSBG 1000 this setting can be found under VOIP\GW and IP to IP\DTMF and Supplementary Services\Supplementary Services. Change the Hold Format parameter to "Send Only" if it is currently set to This parameter controls if the gateway should expect a SDP with fields set as a=sendonly and c= containing the client's IP address, or if it should expect a=inactive and c= Setting this value to Send Only allows the Lync PCs and phones to successfully pass music on hold to a PSTN caller.

One-Way Audio with AudioCodes and Encrypted Lync Media

This is a quick one: If you're using media encryption with the latest MSBG 1000 firmware posted to the AudioCodes website, you might find yourself experiencing issues with one-way audio. Specifically, Lync users are periodically unable to hear PSTN callers. The issue is sporadic, and may appear or disappear as the call continues, although a hold/resume tends to fix it temporarily.

After doing some packet captures you will see a bi-directional RTP stream between the gateway and Mediation server, but the Mediation server does not actually pass any RTP packets to the Lync endpoint side because it is unable to decrypt the media.

This is a known issue in the 6.40A.037.009 firmware and can be resolved by upgrading to version 6.40A.059. You'll have to call in to support to retrieve the newer firmware since the one with the issue is the latest posted to the AudioCodes website (and unfortunately very dated, seeing as it was released in May of 2012.) Happy upgrading.