Advice Wanted : Mobile Phones

I'm thinking of getting myself a mobile phone for when I go back to work (so that I too can announce "I'm on the train!") However, I know next to nothing about how to choose one (I've never bought one before).

My criteria are:

Use : To ring out in emergencies and semi-emergencies. I'm not interested in deals that provide chat time, I have a land line for that (it'll normally be switched off).

Features : Just the whole "phone" thing. I don't care about cameras, ring tones, text messages, games, etc (although, I'm mostly interested in the cheapest option so if it's cheaper to get a phone with them, that's fine). I also want something that would fit in my bag, but frankly anything smaller than a brick works (also I find the really small phones annoying).

Main want : Something cheap.

Any and all advice appreciated!