2 months ago I have ported my landline to 2talk and I am using only my iPhone 5S and the 2talk connect app to receive and make calls. 95% of my incoming calls drop under 1 minute or don't ring at all. My outgoing calls are with an echo and drop sometimes or don't connect at all. I am connected to the internet with a FritzBox 7360 / 2 degrees Any idea if there is something I can change?
If you have a DECT cordless phone, try registering it to your Fritzbox and then using that as the SIP endpoint instead of using the iPhone app. That will remove wifi from the equation and maybe cellular 3G fallback too, plus any issues on your phone.
This will allow you to determine if the link itself is OK (presumably UFB or xDSL?). Then you could take it another step from there depending on the outcome.