Lync uses a feature called Comfort Noise that reduces network traffic in moments of silence, but still allows the voice gateways to generate some white noise to avoid the “hello? are you still there?” conversations. If your gateway is not configured to support Comfort Noise, then Lync will throw Event ID 25073 on your calls, saying The Mediation Server service has received a call that does not support comfort noise […] The Trunk does not support comfort noise.
If you’re using a Cisco router as your voice gateway, you can enable Comfort Noise support by using the following command under your voip dial-peer connecting into Lync:
rtp payload-type comfort-noise 13
Or if you’re using an AudioCodes gateway, you can find the options under VoIP > Media > RTP/RTCP Settings, but make sure you’re using the Full menu set.
After enabling Comfort Noise support, you can run a packet capture and notice the RTP packets showing support