2 talk connect in and outgoing calls drop

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 degreesAny idea if there is something I can change? Thanks