The phone I've linked to is SIM-free, Jennyjoan.
That means that you can put any SIM into it and it will be the SIM that determines how the phone's use is paid for. i.e. if Maggie has got a SIM linked to a Virgin contract it will become a Virgin 'contract' phone. However if she's got a Virgin Pay-As-You-Go SIM it will become a Virgin 'PAYG' phone.
If, as I did, she were to put an Asda PAYG SIM into it, it would become an Asda 'PAYG' phone.
Anyone who buys a SIM-free phone can choose BOTH the service provider they want go with AND (where that provider offers both types of account) whether to opt for a contract or to use PAYG.