Sim Cards Phones

Can you use pay as you go sim cards on contract phones?
I want to buy an lg viewty or lg KF700 off my friend but hers is a contract phone. Can I use a pay as you go sim card on the contract phone? Also, if my sim card isn’t the same network as hers, do shops give out sim cards for free? Thanks in advance!
As far as I know it has a good chance of working as long as you have the same provider, or you might be able to regardless. I just bought a Tmobile prepaid phone for my mom but put her contract sim in it. If you have different providers, your phone may need to be unlocked (just google “unlock [insert phone name here]“). To save yourself some time, just put your sim in it and try it before you buy it!
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