How to unlock an iphone 4s which has been reset?

I have just bought a 4s from a friend. I reset the phone for a fresh start and, while going through the setup process, the phone doesn't recognise the sim. The original owner is going to get an unlock code from the old carrier (3) but without a 3 sim to put into the phone, how can I get through the setup process and unlock it?

I don't have access to my friend's old sim BTW.

You can't.

You don't need a sim from the original network (3 in this case) to unlock the phone. You need a sim that works on any other network, eg Vodafone, Orange, etc to do it. The sim needs to be the correct size for the 4s (a micro-sim). Get the unlock code from your friend when she gets it from 3, and put in the new sim. The phone will ask for an unlock code/subsidy key or something like that. Enter the code your friend gives you, and the phone will run the process to get unlocked. Once this is done, the phone will be unlocked to work on all networks, including the original network, the network whose sim you used, and any other network you may like to use it on.

3 will charge your friend a fee of £15.32 to unlock the iPhone, and the iPhone will need to be on IOS 5 or higher, to be able to be unlocked. The information you need is at the link below:

http://support.three.co.uk/SRVS/CGI-BIN/WEBISAPI.DLL?Command=New, Kb=Mobile, Ts=Mobile, T=Article, varset_cat=billing, varset_subcat=3772, Case=obj(4363)