Why won't my 3 sim work in my Sony Ericsson mobile?

Right now I have an 02 pay as you go sim in my unlocked Sony Ericsson phone it works fine. I've put in my 3 contract sim in and it comes up on the screen "invalid sim". I'm using a very old phone as mine was stolen.

From looking at the sims, 02s has a large chip and 3s is quite small so maybe the phone can't read it? If that's the reason can I request a larger chip from 3? This is second sim I've requested from them after they stupidly sent me an iPhone one.

Sorry but the 03 will not work in your phone. You need either a newer phone or an 02.

You say it is an old phone, and that could be the cause of the problem. 3 network sim cards don't work in 2g handsets, and the phone you have may be 2g only. Another problem based on your description may be that 3 may have sent you a micro sim or a nano sim, neither of which will fit in the sim slot on a standard phone. You need to request a standard sim from 3, and change your phone for one that is 3g capable. The replacement phone must be either unlocked, or locked to 3 network. Some smartphones that are not iPhones will need a micro sim (only the iPhone 5 takes a nano-sim). When you replace your phone, the company you buy from will be able to advise you as to which type of sim the new phone needs, and supply one of a suitable size.