There will be a difference between a personal mobile and a corporate mobile and could be down to how the government/security services handle mobile devices issued to the officers of state. I would imagine there are strict procedures in place when someone switches from a normal MP role to Government and also to one of the great offices of state and i would imagine that there would be stricter removal and cleanup policies when a person switches between them. Rishi will have gone from chancellor of the Exchequer --> Normal Member of Parliament --> Prime Minister since COVID, I assume this will have meant mobile number changes and security level changes between each move.
Keeping the same number and leaving a history available for the prime minister every time it changes would make it easier for foreign spy services to follow them, and make it harder to identify the source of when an account is hacked (if it ever is).