Probably for international calls, since they charge extra for calls to numbers outside the US. I.e. if you use your US-based phone in Canada to call Canada, you may end up paying both the roaming charge and the call-to-Canada charge. Better to text message if possible, at least with T-Mobile. Or keep a Canadian SIM card (prepaid or postpaid, depending on which is best for your usage) for when you roam in Canada.