Still not much information for these so here ya go
If the phone needs unlocking I recommend:
The only other things you need to (and can) do, are to make sure the network and internet settings are right and to change the voicemail number.
Go to Settings > Mobile network
make sure 3G is turned on (as long as you want it to be) and network selection should be 'automatic'
Stay on that menu and tap 'edit apn'
Enter these settings:
APN - giffgaff.com
Username - giffgaff
Password - password
Now you just need to change the voicemail number:
Go into the 'phone' menu and then go to call settings.
Tap 'voicemail number' and change it to 443