I have a 15 quid PAYG punting phone from Tesco which works just fine. The only exception has been one touring girl that wanted me to email her when I arrived so she could tell me the room number. I had to explain that I only had a very basic phone that didn't do email, which made things a bit trickier as she wasn't keen on giving out the room number in advance or her phone number. So I'm considering getting a slightly more upmarket punting phone.