Does Three phone company lock iPhone's? (UK ONLY Please)?

My friend just brought a new contract less than 3 months ago but will be selling the phone onto one of our friends. She has defaulted on payments already. Will Three.co.uk block her phone? Please answer if you have experience or know for definite whether they do block phones when payments are not made.

Thanks.

Added (1). Thanks for the answer but I mean the phone not the sim card. Do Three block the actual iPhone?

Yes, indeed, they do, so the new owners
need a new sim number/ card of their own.