iPhone not working with my SIM. It is with other SIMs

Hello.
Picked up a 4s 32gb today to pop my sim into because work have upgraded us from Samsung galaxy to blackberry. After 2 days of trying to fiddle around with the BB, I have come to the conclusion it is awful. Hence why I went to get the 4s (awaiting 5s later rather than buy 5 now)

Now here's the issue. My SIM card is T mobile, and so is the phone (well EE now a days) but when I put in the card it doesn't recognise it. Stating it is unsupported for activation.

I trust the retailer, but the only reason I can think of for it not to work is if the phone is locked to a different network. I am going to visit them tomorrow to address the situation but was wondering if you guys/gals knew of anything that could prevent the sim from being recognised apart from a dodgy phone. I.e. is it possible that my company have asked the network to prevent the card being used in iPhones specifically?

N.b.
I have tried the sim in Samsung/Nokia/BB phones and it works. Put it into two different iPhones (only one was supposedly EE. The other was my wife's vodafone) and it didn't work
Extra n.b. my wife's vodafone sim works in the allegedly EE iPhone

iOS 6.1.2
Thanks in advance