This requires a bit of a history lesson:
My network has 2 Cisco routers (one is a 4331, the other is a 2921) that forwards SIP traffic to our service provider. One handles inbound SIP; the other outbound SIP. We control the internal routers, but the destination router is a service provider router. A few months ago, due to a bad IP address in their config, we lost all outbound calling when we took down the outbound router for maintenance.
My boss wants me to come up with some way to monitor SIP traffic in PRTG. The problem is that the SIP sensor in PRTG requires the credentials for the SIP server. This is something that our service provider immediately shot down. Is there any other way to monitor SIP traffic without knowing the SIP server credentials? We currently measure throughput on those links, but it did not show as down when the failure occurred. The traffic was going out just fine, it just got dropped on the other side.