That 7100 feature is not available on the UC Server. If an assisted transfer is used to send the call out to the PSTN (cell phone for example) there is not a way to retrieve it. The 7100 stays in the call path for enhanced FMFM, which is why it can still return the call even if it does go out to the PSTN. This may not be an option for you, but if the call ended up on another extension registered to the UC Server it could be picked back up from your desk phone by dialing *27<extension that currently has the call>. This as well as some other supported SPRE codes are listed in TN117 - NetVanta ECS and BAS Feature Codes.
That is what i thought, So what I suggested to the EU is that they keep FMFM turned on so that they will get the calls that way up front before it gets to the UC. Thank you