Lync 2010 does not support SRTP Key Derivation.
It is not enough to set the key derivation rate to zero, it must be disabled.
If the flag KDR is shipped in the SIP SDP crypto field, then the call will not established.

The Windows EventLog will show the following error:
Source: LS Mediation Server
EventID: 25039
Message: Gateway did not offer SRTP keys which is required by Mediation Server.