When you say "unlock my SIM" do you mean "unlock my phone"?
You need an unlocked phone to work on 48 so you need to find out if this is locked (and to what network) or unlocked.
Where did the iPhone come from originally?
What happens when you put your 48 SIM in? Any messages/errors?
You could try this and see if it helps:
- Put the 48 SIM in
- Make sure that you have a current/valid 48 membership on your account
- Make sure that you have a small amount of cash credit on your 48 account (e.g. €1)
- Sync with iTunes
If the phone is not locked to another network this may activate it on the 48 network.
The credit is required because this process can send a UK SMS text (or maybe more than one?) and these are not covered by membership allowances but cost 25c each.
If that doesn't work then it may be locked.
If necessary contact 48 support in case they can offer assistance:
How to get help
Hope this helps.
