Hey I just recently purchased an iPhone 4s and I want to know what's wrong with it?

I bought an iPhone 4s of Craigslist and everything seems to be working fine. The guy before the purchase told me that the phone was unlocked. However, when I went to my carrier (Bell) they cut my sim into a micro-sim but the phone read it as No Sim. The sim already in the phone when I purchased it read No Service. The micro-sim that was already in it was from Fido. Then I tried to put a micro-sim that belonged to Rogers but didn't work now because our contract with Rogers was over. The phone read the Roger's sim as No Service as well. So what's the problem? Is the phone locked? Or could it be my normal sim that was cut into a micro-sim (Bell)? Any help would be awesome.

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