Obtain Basic Information

• Numbers involved
• Extensions involved
• Time of the call

Check PBX Call History

• Check the domain's call history. If the call is hitting our switch then we would show an entry in the call history page.  
• Open and review the call trace for any failures (see Obtain a Call Trace). Provide it to your service provider if necessary for further troubleshooting.
• Look for error codes in the 4xx range (see Common SIP Response Codes). The most common issue is that the phone is not configured to authenticate against the proxy.

Check Phone Logs

Most phones have their own logs, but have to be turned on. Yealink, Grandstream and Cisco have specific sections for logging. You would turn these to a functional level of DEBUG or similar. It is safe to leave this on for a few days until the user experiences the issue again. Once they do be sure to turn logging off as it does stress the CPU of the device. The log function usually performs circular logging which means it will delete the oldest entries to make space for new ones. Don't wait too long to view the necessary logs. While review logs you'll want to look for errors such as NOT FOUND, BUSY HERE, PERMANENTLY MOVED, or ACCESS DENIED. This will give you clues as to what error is causing the failed calls.

Check Packet Captures

Many phones are capable of performing packet captures, as are most firewalls. This will give you a complete view of the traffic between the phone and our servers. Thankfully you do not need to review all of the traffic. You can limit the traffic by type (I.E. UDP/5060) or by IP address (see IP Addresses and Ports). You should see two-way traffic. If you are only seeing traffic one way it is an indicator of traffic filtering somewhere in the chain. The firewall is most likely. 

Ask for Help

If you still can't find the source of the issue just send us a support request via 611 or email. Remember to include any information you've gathered using the above steps.