I’m wondering how feasible would it be to just use a SIM card from another country (e.g. in Estonia, you can get a prepaid card for 1 € that works in EU roaming just fine, with domestic-like prices on local calls). How many services in Germany require you to use specifically German number?

The EU roaming thing usually works for 6-12 months until you are required to connect to the home network.

I don’t think that’s a big problem though? Especially if you live in Germany and get a SIM card in e.g. Czech Republic.

It depends of course how far you are. I used to use an orange Spain SIM before the EU roaming deal because they had free roaming on sister networks. But I didn't go there so much.

Several do require it.