I'm also trying to get an German 1&1 VoIP number to put in operation on my iPhone 4 with the latest Zoiper app.
The number is in the router at home *not* activated.
I've also the registering problem. Soemtimes more, sometimes less. In-Calls are (mostly) not routed finally to the app. Out-Calls are nearly always without audio in both directions.
With my Sipgate account everythings works perfectly!
Any hints how to set up Zoiper manually with 1&1?
Thank you and best regards