I can only comment from the UK, Are you using ISDN/analogue or SIP for your external lines?
If you are using ISDN, i would recommend the CCS TRACE ENABLE CONTINOUS under Maintenance Commands. For SIP, you need can use the SIP TCPDUMP ON/OFF commands under the same form.
When using CCS TRACE, make sure you turn it off once you have finished your trace, using CCS TRACE DISABLE.
That's where i would start.