I just went thru 3 days of hell with Tracphone trying to add minutes and service days from Canada. I knew the phone would not work in Canada so I used their internet site to do the updating.
I tried using 2 credit cards including the one I used to purchase the phone from their site.
4 emails and 3 calls from their reps and the problem explained: "it was an executive decision not to accept international credit cards to add minutes and days."
So the only solution is to get across the border, buy up some cards and use them when needed.