[cisco-voip] what to debug when calls disconnect "normally"?

[Kelemen Zoltan]

We have a client with CCME 4.3, 7940 and 7911 phones, and ISDN PRI 
complaining about some calls being dropped prematurely (they claim only 
calls to a certain, third party provider, not the one giving them the 
ISDN line).

However, debugging isdn q931 shows only Cause code 0x8090, "normal call 
clearing" for all the calls they've mentioned as prematurely disconnected.

While I tend to believe it's a problem on their provider's side, it 
still disturbing, that the router is the one initiating the disconnect, 
not the provider. The calls can last anywhere from 0 to 20 seconds, but 
always disconnect normally.

Any idea, what to debug now, to find out more?

thanks,
  Zoltan