Hi,
Yes you can do it using Cisco voice gateway. There are many different scenerios. In many of them you should use these features : dial-peer hunting.
You define 2 dial-peer. First voip dial-peer uses gatekeeper (session target ras) , second uses pots . Using preference command you can adjust hunting order (priority) in addition to dial-peer hunt command.
If you are using analog connection between PBX-router you probably need additional port to sent voice to PBX back. In this case you also can control gatekeeper IP address to probe it but this is not actual RAS probe onlt ICMP probe (busy out feature ). (Thi feature is used to busy the port if IP address is not reachable)
If you are using PRI you can use additional channel to send voice back. In addition to that you can use isdn network-failure command to send a failure code to PBX so PBX can route the call to the second route on itself. But this may require that PBX may understand this code. Nortel do it.
Regards
