Basically everything is VoIP these days (VoLTE, etc.). Online services sometimes have secret lists of phone numbers they don't like, but "voipness" isn't a silver bullet for determining whether or not a service will like your number or not. The JMP people wrote more about this at https://blog.jmp.chat/b/2022-sms-account-verification
Anyway, this thread is about Signal, and JMP numbers work with Signal, which is why I suggested it.
Why do you need a German phone number? Many countries let anyone have a phone number, with no proof of address or other identifying information. Just use one of those numbers instead. One example service is https://jmp.chat/ but there are many others.
Anyway, this thread is about Signal, and JMP numbers work with Signal, which is why I suggested it.