the below works for an iphone on the O2 network (UK), but i suspect that other networks do similar.

if you have a locked iphone, it is now possible to get it unlocked so that you can use any SIM. it's free to do if you're on anything apart from a pay as you go tariff (PAYG unlock is 15 quid).

you just need to go onto the O2 website and register your details to get it unlocked. takes up to 14 days. if you are in the middle of a contract, then you will, of course, still have to honour the remaining payments, but can still get it unlocked.

once you're unlocked, you're good to go to use a Swiss prepaid SIM.