There's already an account with that name- Can't work out why I can't register

Question

Hello Community,

I want to set up Zoiper on my Lumia 950. In our apartment we use PhonerLite on our desktop computers and they are functioning parallel very well. Only with Zoiper I get the message: There's already an account with that name. Therefore I think it's not a problem on the provider side.

I checked the settings multiple times and couldn't find the problem.

I hope you can help me.

Greetings