My mate bought a 2nd hand iphone3G and is wondering if his old sim card will work in it.

I have an iphone 4 and know its a micro sim. Not sure of 3G.

Also, can he continue using his PAYG "tariff" or will he need to contact o2 to move onto an internet/data type deal?

If the iPhone was originally on O2 or is unlocked yes it should work. As for the tariff that all depends on what he currently gets.

The 3G took the more standard mini-SIM so that shouldn't be an issue.

How old is the SIM?

I had issues with an older 02 SIM which was supposedly 3G enabled, if his SIM doesn't have 3G marked on it or has a serial number beginning 8944110063 and has 3GD2+ marked on it he should get it replaced by calling 02 CS or going into a 02 shop. That is if he wants access to the 3G network.

As your mate is on PAYG and not tied into a contract I recommend he checks out giffgaff. You can get a 'goodybag' which gives unlimited texts and data and then he can choose how many minutes he needs. 250 minutes is £10. It is a 30 day rolling contract. giffgaff uses o2 network so if phone is unlocked or locked to o2 it will work.

yes sounds good.

whats the members loyalty/referrals thing all about?

this was my main issue. I'll see him tomorrow, thanks for that.

giffgaff is owned by o2 but is unique in that a lot of support comes from the 'community' rather than them having contact centres. Means they can keep their costs down and their goodybags cheap. I think members get points (points=cash) for helping on the forum. Referrals are for existing customers 'recruiting' new ones and also get cash for that.