Most mobile phone shops these days are commision based...or at least have target based salaries....you'd maybe have to sell 50 contracts per month or so.
From my own experience as a customer in these type of shops, sales people are very keen to show you the latest models, when they find out your only in for a memory stick for example, their helpfulness and interest stops.
Phones4u are quite a large well known company so maybe a better choice of employer than some others.
What dmsymr said x 1,000,000,000. A friend of mine left the T Mobile shop where he worked after the sales targets started getting ridiculous! It's a VERY high pressure environment. He also felt that he was pressurized into selling people what they didn't need, just because a certain price plan or phone was "flavour of the month"