If you have a SIM card in your US phone and it's unlocked, you can probably also just by a prepaid sim card and drop it in. Make sure your phone is compatible with the network the SIM is from--we have a couple of carriers here who use odd bands (e.g., Wind, Mobilicity, and Public Mobile - not all phones have the frequencies they are on). Then forward calls from your US number to your Canadian number so if people call you your phone will ring in Canada, and you'll pay your US carrier's rates to answer the incoming calls, plus your minutes on the local sim.
If you are MOSTLY going to be making calls from somewhere you expect to have internet access an alternative is to set up some kind of WIFI calling on your US phone, and dodge the mobile network. This won't work when you're out in a pub or something though, but it might work in your hotel and at the office, etc.