This app requires mobile data or wifi
this does not fix his problem because if he WiFi they wouldn't have been roaming to begin with.
If he used the app, he would be roaming, incurring data charges.
Can't tell if you didn't logically think this through or you are pumping your own app.
When I goto US, from Canada, I use T-Mobile Sim for unlimited calling and data 3$ a day...
If you are a US resident you have plenty of carrier options that cover unlimited entire North America coverage.
Also the OP could have cached/saved the area in google maps or installed another GPS/navi software that has a offline map mode.
Now to the article... Should have planned ahead. So many travel addons from carriers, using a SIM card from country you are traveling to... Which I know is available at the exact border crossing shops at the border that the OP went through
I have a (physical) device called a FreedomPop. It is a local wifi hotspot that routes traffic over a mobile data network. And it's awesome for giving the middle-finger to overpriced hotel wifi services and the like.
www.freedompop.com (I have no connection other than a very satisfied user). I'm so satisfied that I won't even post my affiliate code, lest someone think I'm shilling for my own benefit.
However, these comments are about providing a wifi hotspot that routes over the data connection rather than VOIP.
The only way I would go back is if T-Mobile suddenly ceased to exist.