Setting up Sip Trunks can be a tricky process. I’ve had my fair share of issues setting them up. Using sip over an NBN or fixed connection with a Static IP is the best case scenario. However recently i have had to setup some phone systems over a Mobile Connection.
- The first thing to do is check that your carrier does not block Sip traffic on their Mobile Network. In Australia Optus would not work for me, I thought this was because of the Carrier Grade NAT. However we now use spintel and their Mobile service does allow for Sip Traffic to pass through.
- Get a good full feature LTE Modem. I have tried to use an Optus modem in that past with no success as the features are very limited.
- On your phone system, change the length of the registration time. This is important because if your IP address changes, then the phone system needs to let the service know the new address. The registration time on most systems is around 30 minutes, I would recommend changing this to 2 minutes (120 Seconds). This should stop the service from cutting out periodically. On my Yeastar S-20 this is done in settings -> General -> sip -> Default incoming/outgoing Registration Time
If you need more advanced features such as using remote (offsite) Extensions and a sip phone app, you will need a static IP address. The best carrier i have found is Zettanet, they offer a Fixed IP address over 4G at a pretty reasonable price. We use them in conjunction with their Simtex Sip Trunks.