Random call Failures due to timeout - "request timeout"
If your phone randomly fails to connect, ringing occasionally and failing other times, it is likely because your IP Address may be changing too frequently. This is common when using 4G services for your internet connection.
One solution is to reduce the server timeout on your SIP phone.
This example will focus on Yealink Phones - if you have another make/model of SIP phone please consult the manufacturer's documentation
Find the IP Address of your Yealink phone, using your computer's internet browser, type the IP address into the address bar to access it's login portal. Note that the default username/password is 'admin' and 'admin'
On the account experiencing the issue, find the Server Expires entry and reduce it to 60.
This setting will force the device to re-send it's IP information every 60 seconds. You can experiment with this number between 60-120 to help improve stability.