I was traveling to Germany a month ago and read that "By law, all SIM cards in Germany now need to be registered on your name verified by your ID document (and an address) to be activated"[0] and "many more places sell SIM cards, than are allowed to register" and "newly purchased SIM card... doesn't work as you still need to register."

I then installed Airalo app and bought an eSim in five minutes, still at home. Once I landed, I had 4G service for a week.

On my trip, I was never asked for my (German?) phone number but had received multiple SMS verification codes to my regular line.

Your experience may naturally be different.

[0] https://prepaid-data-sim-card.fandom.com/wiki/Germany