Q: Iphone 4 unlocked - No Network with New Sim

Help Please! After being given my sisters old Iphone 4 we proceeded to get it unlocked with the provider. Confirmation was received on this yesterday by text and email. I ordered a new micro sim from my provider which arrived yesterday so last night i plugged the iPhone in to the iMac to sync it with iTunes. Tried and it acknowledged my phone name (as id set up my apple id) and said for me to put my phones passcode in to open the phone and try again. Did that and tried again and it said it didnt recognise my device (or words to the effect of). Contacted my provider who advised that my Sim was not active yet so they set that in motion. After 2 hours i tried all this again and still no luck with the phone showing "No Network".

I contacted my provider again and after ages they said they couldnt see any issue with the sim and that the phone must not be properly unlocked (this was 10 mins before their chat service ended mind so not sure if just wanted me off the line). We contacted the phone provider this morning and they confirmed through the IMEI number the phone was 100% unlocked so thats not the issue!

I have reset the network settings - nothing

I have switched Airplane Mode on and off then the phone off and on - No change still No Network

The Carrier still shows as the original provider but does not prompt an update when i am in there so i dont know how that can be changed?!

HELP!