We have connected two Siemens Hicom 350 PABXs using E1 controllers on two routers. The routers are connected via an IP (over ATM) link. Siemens digital phones are connected to each PABX. Calls are going across smoothly, however, when callback feature is initiated from a phone on one PABX to a phone on the other PABX, the initiating phone shows the message "Not possible" although the callback is actually set and does call back. The message should show as "Will call back".