Just wanted to add and FYI. Many US phones are only useable on the network they are made for. IE: sprint or verizon, etc. Also, many of their phones don't have access to the different bands. IE: 1800, 1500. Something, I learned a couple years ago when looking to buy in the US and use here. Also, as others have said, if the phone is locked to a carrier, it won't work here.
If you happen to be lucky enough to live near the border, I'd just buy a prepaid sim for AT & T or T-mobile as that is the easiest and has options. If you can, Ultra Mobile has a tourist card, good for Canada-USA-Mexico good for 21 days. Data, text, calls $30 US. They also have a $19 US 30 day plan that's almost as good. Both are prepaid and include the sim card. AT & T is the most expensive, but it is also the only one that we never had any issue with at all, but it was $50 US for 30 days.