Can a non-iPhone contract and SIM card be used in an iPhone 5s?

I currently have a contract with Orange/EE for a Blackberry 9790 phone, with a standard size SIM card.

If I bought an UNLOCKED iPhone 5s handset, and ordered a nano SIM card with my … Show more

The phone has nothing to do with the contract, you can use PAYG or contract SIMs in every phone as long as it is unlocked to the SIM cards network.