Sim free applies to the network only O2 / voda/3/tmob/virg/orange etc ... not the contract type (payg or contract) ... you will be fine ...(IMO) ... it use to be the case that Carphone Warehouse O2 payg phones were all unlocked (will take any network sim) ... worth checking ... buy it payg not sim free, take the payg sim out and replace it with yours ... but understand if your network doesnt supply the phone they may not support any special serices that come with the phone and you will probably need to obtain the sms/mms/email settings and enter them manually ...