I have centrex phone lines connected to FXO ports at one of my remote sites. When someone calls in they hear two ringback on their phone before the IP Phone starts ringing. I need to shorten the discrepency in the ringing. I don't expect it to by syncronized but I know it can be closer. I'm running H323, using CallManager 3.3(4) and running SRST on the router. I've include the relevent part of my config.
The router is waiting for caller ID information, which on FXO is delivered between the first and second rings. After it receives (or doesn't receive) caller ID, it goes ahead and extends the call to the destination.
If you're willing to sacrifice caller ID, you can set "no caller-id enable" on your voice-ports, but if you need caller ID, this is basically something you just get to deal with. It's not Cisco-specific, that's just the way caller ID works on POTS lines.