So you think about changing mobile phone operator, and you see offers of a free PAYG SIM. Sounds OK. You get your phone unlocked and send off for a SIM. The SIM arrives, you buy some credit for it, and then discover that it doesn't work in your old phone :-(
Turns out that what you thought was a SIM is probably a USIM - something new and slightly different, for 3G phones. And not all of them work in older phones.
I have a Nokia 3330 phone (this is only 2G), which works OK with Orange 3G SIMs. The phone also works OK with a Virgin 3G SIM obtained a few years ago. Put a new Tesco SIM in the phone and (unless the battery is absolutely fully charged) the phone turns itself off within a few seconds of being turned on :-( Same with a new Giffgaff SIM, and a new O2 SIM. Tesco and Giffgaff are both virtual operators on the O2 network, so maybe it's not surprising they behave in the same way as an O2 SIM. Put a new Vodafone SIM in the phone, and the phone works a little better - seems to cope with the battery being slightly flatter. Put a new T-Mobile SIM in the phone, and it's absolutely fine. So there's something definately odd about O2 SIMs in older phones.
Moral - if you have an old phone, you really need to try a new SIM out before loading any credit onto it.
Moral 2 - if you have an old phone, it's better to steer clear of O2 or any virtual operator on the O2 network. SIMs with numbers starting 894411 are on the O2 network. I haven't tried asking O2 if they still can supply 2G SIMs.
The above are my findings on a Nokia 3330. A friend has a Nokia 3310 (that's the previous model) and has no such problems.