One way you can tell whether you are being subjected to roaming charges while traveling in Canada is to check your carrier when you open your phone. It will tell you which carrier you are accessing to make the call. If you see Rogers, beware; it is definitely a Canadian provider and could result in very high-cost calls.
Avoid Roaming Charges
Options to avoid this kind of roaming charge include keeping your minutes low or buying a minutes card from a Canadian provider. Another option is to contact your cell phone provider before leaving for your trip. While relatively few and far between, there are U.S. providers that include Canada within their area of service. Still others allow you to add on a service package for your trip that eliminates or reduces the roaming surcharge.
Forwarding Not Solution
Also beware of forwarding your calls to your voice mail. On first thought, this seems like a good way to avoid answering an incoming call; but this may be considered answering a United States-to-Canada call. Worse yet, you may be charged again when the call is forwarded back to the United States, the location of your voice mailbox.