Agree with the above, check if the unit itself is crashing via the uptime counter.
Then the next step will be use of the syslog feature to log why each call is ended. Syslogging is straightforward to setup, free syslog servers listed
here, but interpreting the log is a grey area.
I'd suggest unless you can figure out something yourself from the syslog, you create a support call and mention you have a sys log. It's hit or miss posting the results on here, first you'd have to post the right entries here, and then someone has to know how to interpret the entries.
Lastly, on the DSLR VoIP forum someone has recently posted that wireshark has a built in viewer for SIP and RTP transactions. Haven't figured it out myself, but it's on the list of things to play around with sometime.