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.