We're having a specific problem with voicemail being hosted on Exchange 2010. We're using Call Manager 7 that connects to Exchange 2010 over a SIP trunk (we're not using Unity cause it crapped out on us). For the most part this setup works fine for voicemail, mwi, etc.
The particular situation where it falls apart is when a call is transferred using the direct to voicemail pattern. At this point I begin to see RTP packets sent from Exchange to the CUCM router with invalid header checksums and on the user end they are prompted by Exchange with "Are you still there?" as they try to leave a message.
Occasionally calls made using the direct to voicemail pattern will go through but there is no discernable different between the calls that fail and the ones that succeed.
Exchange 2010 is running on server 2008 R2 with rollup 1 installed. The Windows Firewall is set to allow all connections on the domain network over which the SIP trunk is communicating. It is also running on a VMware WM though I don't know that this would make any difference.
Any suggestions would be appreciated and I can provide any debugs or traces that would help.