In the UK mobile networks are required to offer number portability[1]. I don't know if that means a mobile network can take your number away, but just like managing the registrar on a domain, you can manage the portability of your number. [1] https://www.ofcom.org.uk/phones-telecoms-and-internet/inform...
But just because you can does not mean people want that. Before it became normal to auth everywhere with phone numbers i happily changed my number yearly.