always mention how your setup is deployed, as you refer to an older posting a link to it does not harm neither.
Also list the involved components (incl. kind/type/endpoints, software versions, ...)
Also how the call flow looks like, what kind of call it it (audio/voice sip/h323), if/where interworking is done, if it happens in both directions, ...
Logs (as well as debug logs) and screenshots do not harm neither.
In general if something strange happens, check your firewall for L3 functionality, on call setups.
This sounds a bit like media negotiation or signaling issues.