new 3 sim in restricted access phone?

From Nottm,

3 have restricted access to my contract phone because of late payments.can i buy a 3 sim and just use it in the phone as pay as you go? my friends payg sim works in it, but do i have to register or something to enable/top up my new sim,and when i do, will it not let me, so wasting the cost of the new sim i intend to buy?