I guess that's Facebook's problem now. WhatsApp uses phone numbers as logins though, doesn't it? Shouldn't be too hard to map providers to different countries.
Likely they'll do it based on the phone number prefix +49
I sense a new market. German number-as-a-Service...
This will be on Show HN in 2 weeks, calling it now.
Get one on Twilio, set it up so you get the verification code, finished. Right?
I would think it would be either by German carriers or else country code of the phone.