Flexisip

I have previously built cellular gateways for Asterisk out of EC20 and EG25 modules (Using EC20 Module, Asterisk, and FreePBX for SMS Forwarding and VoIP, Connecting a DJI/Baiwang Enhanced Transmission Module to Asterisk and Telegram SMS Bots), and used Flexisip to fix SIP clients missing calls in the background (Adding SIP Push to Asterisk/FreePBX with Flexisip and a Custom Linphone Build). One piece was still missing: some SIM cards simply do not work well in a module, and the easiest fix would be to let an ordinary Android phone act as the gateway itself. ...

Back in 2022, I wrote a post about using an EC20 module with Asterisk and FreePBX for SMS forwarding and VoIP. The setup could already route calls from the SIM card in the EC20 to a SIP client, but the post ended with one fairly serious usability issue: the SIP client on my phone had to stay running. Otherwise, incoming calls could not reach the client, and the caller would hear a busy tone. At the time, the only easy solution I found was Zoiper Push Proxy, which was not free. ...