At the risk of resurrecting this thread, I'm trying to do the same thing.
I've configured my OBi110 to route incoming calls to my IP phone. But sometimes I want to transfer the call back to the OBi so someone can pick it up on the Phone port. The way to do this with my IP phone is to set up a 3-way call, which works, then press the transfer button to join the two parties, which works for any SIP device I try to transfer to
except the OBi itself. In that case, my OBi's call history says "End Call (400 Bad Request)".
What am I doing wrong please?