PJSUA2 run app in background without push

Question

I'm making a VoIP application on PJSUA2. I took the application base from the PJSUA2 example. When the application is open everything works perfectly. But when the application is hidded it freezes (in the settings bg modes are enabled: VoIP, Audio, bg fetch,bg processing, remote notification, External accessory). I read that I should use PushKit + push notifications, but this is not what I need. I use asterisk server and tls connection. Is it possible to keep a live tls connection and receive requests in the background? If yes, how?