If i get a unlocked or sim free iphone Can I use any sim?

Okay my moms is supposed to be getting the iPhone 6s and I live with my dad but I supposed to be moving back with her soon the thing is they have different cellphone company s. Anyways if I get the sim free iPhone will I be able to just put a SIM card in it without haveing to change contracts threw the phone

If you buy an unlocked iPhone 6S, you can use the phone with any carrier you want. So I would recommend getting the SIM-free or unlocked iPhone 6S. If you need unlocked or SIM-free iPhone 6S at reasonable price, mail me for more info.