Iphone help please.

[Madders]

I've sold my Iphone 3gs to a chap on ebay who says that he can't now get it to work as it shows  no network. He's on O2 and so was/am I. I went to the O2 shop to trade up to the new 4S and they sorted everything out and I left the shop with a working 4S and a working but unconnected 3GS. So, what's the score? Why can't he get it to work and pick up a network?
I've advised him to speak with O2 or to take it to any independent dealer who should be able to sort out, what I feel to be a very simple problem.


Thanks in advance for your help.


Madders