This is what i get on an incoming call (in syslog)....
08-20-2009 07:08:58 Local2.Debug 192.168.248.206 DLG Terminated 29c958
08-20-2009 07:08:58 Local2.Debug 192.168.248.206 CC:Ended
08-20-2009 07:08:58 Local2.Debug 192.168.248.206 [0]AUD Rel Call
08-20-2009 07:08:58 Local3.Debug 192.168.248.206 [0]FM Alert Stop RxTx (c=0024a404;a=0)
08-20-2009 07:08:58 Local2.Debug 192.168.248.206 [1]AUD Rel Call
08-20-2009 07:08:58 Local3.Debug 192.168.248.206 [0]FM Alert Stop RxTx (c=0024fe9c;a=0)
08-20-2009 07:08:58 Local2.Debug 192.168.248.206 FXO:Stop CNDD
08-20-2009 07:08:58 Local2.Debug 192.168.248.206 AUD:Stop PSTN Tone
08-20-2009 07:08:58 Local3.Debug 192.168.248.206 FXO:On Hook
08-20-2009 07:08:58 Local2.Debug 192.168.248.206 AUD:Stop PSTN Tone
08-20-2009 07:08:42 Local2.Debug 192.168.248.206 [1]RTP Rx Dn
08-20-2009 07:08:42 Local2.Debug 192.168.248.206 AUD

lay PSTN Tone 9
08-20-2009 07:08:42 Local2.Debug 192.168.248.206 CC:Ringback
08-20-2009 07:08:42 Local2.Debug 192.168.248.206 [0]RTP Rx Up
08-20-2009 07:08:42 Local2.Debug 192.168.248.206 [0]AUD ALLOC CALL (port=16444)
08-20-2009 07:08:42 Local2.Debug 192.168.248.206 [1]RTP Rx Up
08-20-2009 07:08:42 Local2.Debug 192.168.248.206 [1]AUD ALLOC CALL (port=16442)
08-20-2009 07:08:42 Local2.Debug 192.168.248.206 Calling:
[email protected]:5060
08-20-2009 07:08:42 Local2.Debug 192.168.248.206 AUD:Stop PSTN Tone
08-20-2009 07:08:42 Local2.Debug 192.168.248.206 AUD:Stop PSTN Tone
08-20-2009 07:08:41 Local3.Debug 192.168.248.206 FXO:CNDD Name=PRIVATE Phone=6XXXXXXXXX
08-20-2009 07:08:41 Local2.Debug 192.168.248.206 FXO:Stop CNDD
08-20-2009 07:08:41 Local2.Debug 192.168.248.206 FXO:CNDD name=PRIVATE , number=6XXXXXXXXX
08-20-2009 07:08:39 Local2.Debug 192.168.248.206 FXO:Start CNDD
If all else fails it could be some sort of corruption. You might try a factory reset and then just datafilling the PSTN line.