Check out ‘devyce’ you can keep a Uk number or setup a new one indefinitely, works on a separate app on your phone, then you can put a local sim or e-sim in you phone for calls data etc… been using without any issues for years without going back to the Uk.
Sorry to jump on someone else’s post, but if you plan to spend 90 days on one the Islands, say Ko Samui, is there an office there to extend the final 30 days? or would you have to visit the mainland?