Which companies offer wifi calling (voice over wifi) in germany?

Hi, I really hope this is the right subreddit for the following question. But because it is a question from germany. Maybe someone knows the answer.

I see that in germany there are many cellular signal providers (or reseller like [sim.de](https://sim.de) i dont know exactlly the name), but I need one that offers wifi calling. I live in the basement where I do not receive any outside signal.

Does anyone know where to get the information which companies accept wifi calling. A co-worker told me that only O2, Vodafone and Telekom (the big ones offer this service) but the small ones do not.

Which really seems strange to me.

Thank you very much for taking the time to read the post

by nonameuser90