Even two years ago, I had to actually change a phone number because I couldn't transfer my number from one provider to their own reseller. O2 Scheißladen.
Huh, I guess I got lucky staying with Viag Interkom and then O2 (which was an automatic switch when O2 bought them) for so long, I only really switched providers in 2020 which was long after the EU regulation was in effect.