Is the call coming in and going out over a SIP-based PRI? I have seen this issue with outside caller to extension with external call forwarding on SIP-based trunks before, it is often related to Early Media settings. I have also seen this issue when the forward destination is a cell phone with custom ring-back tones, never did find a solution as it was inconsistant for troubleshooting and we just gave up... but again this has always been with SIP trunking directly, not via a PRI circuit or SIP/PRI gateway, might not be your issue, just sounds very similar.