If the provider provided the phone at a cheap price then it surely matters little if it is contract of PAYG, they will want their subsidy back, and so are likely to lock it so you can only use their network. Although I can see if you are committed to a contract there is less imperative to lock the handset, since you've committed to a period of payment anyway. As Chuck says, best to get a Vodafone SIM and "suck it and see". Older phones are cheap to unlock, modern "smart" phones not so easy, nor as cheap.