[Users] Parallel ringing

Hello, how could it be done through AVP´s and database lookup, to do a parallel ringing. Following setup: If uri eg example(a)test.dom is called, this call should be forwarded in parallel to a pstn-gateway. That means, receive the AVP with the telephonnumber associated to uri example(a)test.dom, rewrite uri, rewrite host/port and forward in parallel to gateway. I tried "append_branch", but was not able to put in the AVP (phonenumber), rewrite host/port points directly to the gateway. I have no troubles to load the AVP, only to make is possible, that the SIP Client and PSTN Phone are ringing in parallel. Some suggestions ? regards, Andreas