I am with Tesco Mobile which uses the O2 network.

I have no problems with sending or recieving calls or texts when on the island.
When i am halfway across from Liverpool to Douglas the phone screen automatically changes from Tesco to Manx.

Just buy a Tesco pay as you go sim card, for i think about £2.50, top it up and away you go, and don't forget to get your phone unlocked.

Or it could be cheaper to buy a Tesco phone for about £25 complete with sim card. But remember to put plenty of credit on it as the Tesco in Douglas cannot top your Tesco card up because they are run differently from the main land shops, even though they are the same company.
All very confusing.