in the street we call it basha chips, pay monthly sim cards, these are the same sim cards you get with your phone...without the phone and you pay £20 upfront and run out the contract sim as much as you want until it gets blocked, using this analogy my friend you should realise by now that the only phones that are affected by sim cards are blackberry models according to my knowledge, they use a specific stream im not too tech savvy on this subject but i know that you cant just use any old sim card and get bb internet and bbm, so the answer is you can use any old sim card, infact its whats on the sim card is what enables the phones abilities, as your sim card is effectively a bank card... its useless without an account and the account is being controlled by how much can be spent and so on...
oh and even if it isnt sim free doesnt matter, just get it unlocked for free using giffgaffs guide 🙂