Users of a Cisco UCME report that calls lose audio after they are forwarded to...

Users of a Cisco UCME report that calls lose audio after they are forwarded to some users. Calling point-to-point seems to function fine, but depending on the type of phone, some calls lose audio when forwarded. Looking at the trace header that shows:

Allow: INVITE,BYE,CANCEL,ACK,INFO,PRACK,COMET,OPTIONS,SUBSCRIBE,NOTIFY,REFER,REGISTER

What is missing and is a reason that the SRTP stream stops working?