Hi,
We had the same issue.
When CFWD activated and call flow is from inbound route >> CFWD >> external number - signaling is OK and no RTP is going through.
NAT was not the issue and progressinband=yes did not solve the problem either.
In our case - activating “fax detection - sip” on the inbound route for the called DID, seemed to solve the problem for us.
haven’t debugged why it has solved the issue - but if anyone has a problem with this scenario and other options did not solve the issue - give the fax detection a GO!