The jailbreak method for unlocking hasn't worked for a few years. The current method for unlocking without going through your carrier involves buying an unlock on eBay. The cost depends on the carrier the phone is locked to. Unlocks for AT&T phones cost around $20. If your 5 is no longer under contract, your carrier will unlock it for free.
That said, Verizon phones are sold GSM-unlocked. You'll need to unlock AT&T and T-Mobile phones to use them on alternate GSM carriers. Which carrier do you use?