I'm using an iPhone 5s. Don't I need to unlock the phone?

It all depends on what towers you are close to. Which iPhone do you have? With newer models you could just pop in a local prepaid sim and use the GSM frequencies. It's a lot cheaper that way regardless.

I have service with Verizon and I was not getting any LTE or 3G roaming service in Vancouver BC. The iPhone showed "Extended 1x". I assume it's analog. The internet was totally unusable. I had global data service for $25/100MB but it would take forever to consume those 100MB. I was close to the downtown area the reception should be good. Any ideas? It's frustrating not getting good roaming service in Canada. I live in Seattle and plan to visit BC a few times a year.