What would happen if I tried to use a locked iPhone with a different SIM card?

I want an iPhone. And I'm thinking of buying one on Amazon.com. I was looking, and one that is "locked" is much cheaper than one that is "unlocked." I was wondering that if I bought this and put a Straight Talk SIM card in a locked Verizon phone, would it work or not? And if not, is there any way that I could get it to?

The iPhone has a unique sim card. No other sim will work in an iPhone except a sim designed for an iPhone.

It won't work because the sim isn't designed for locked iphone. You could get unlocked iphone or phone so that any sim can work in the world.

Check this page http://www.att-iphone-unlock.com/iphone-no-service-factory-unlock-troubleshooting-guide/

@mccoyblues: There's no special iPhone SIM. For example the SIM I used in my iPhone 2G & 4 was originally used in a Nokia 5100, 6230 & 6230i.

If you bought say a T-Mobile iPhone & put in a SIM from AT&T, it simply wouldn't work. The lock is designed that only T-Mobile SIMs could work in them, unless it was unlocked then any would work.