Just had to change mine, what a pain in the arse.
Had the same works mobile for approx 15 years, then the Pandemic came and so did redundancy.
The telecommunications firm that my old company used, wanted £600 !!! to transfer the number to my new phone.
Needless to say that I didn't take them up on their offer
Not cellphones but...........
When we first moved to Redhill we acquired the telephone number at the house of 62726. Shortly afterwards "they" aded a 7 prefix making it 762726. Quite memorable. We moved house and took the number with us. We moved house again (my late wife was somewhat of a nomad) and B.T. transferred the number again. Then we moved to Cornwall. The number there (B.T. again!) was 333690. Quite memorable.
"Hello B.T., we're moving into (address) which has the telephone number 333690, we'd like to retain it please".
"I'm afraid that that's not possible"
"Why, we've done it several times before".
"We're not allowed to reserve numbers".
"How did we manage it where we live at the moment"?
"I can't comment on that".
But I wrote a letter to the main man and yes, we got to retain the number". It's pointless ringing it because my tenants have had it disconnected.
There was a time when each network had it's own prefix ! If you changed network your prefix changed. Now if you change network you can take your whole number with you. Plus side is you never need to change numbers ! Minus side is you don;t know what network you are using when you ring someone. I have free calls within my own network, but recently my mate changed networks, and I noticed my bill was almost double it;s normal amount. I rang them and they told me I had exceeded my limit on other networks limits. It was my mates number that did the damage.