After you provide your gov ID
https://www.comparitech.com/blog/vpn-privacy/sim-card-regist...
So now this phone number is tied to your gov ID and bank account, amazing design of a single point of failure based on a broken protocol (GSM).
And the SIM registration requirement in nearly every EU country exists for 10 years now - in my case it was as simple as replying with code to operator's message because they had my personal information already for over a decade. There was a grace period after which unregistered SIM cards become dead - the requirement was dubious but you had to comply in order to call, text. There were "solutions" I've bumped on in depths of the Internet but neither felt serious nor safe.