If you are not using the phone a lot, this is what we do:

We purchased an AT&T sim card to put in our iPad. It is a monthly plan but you can cancel anytime and the card is always valid if you need to "re-activate" it.

We use our iPad to make calls with either the wi-fi, or 4G (from iPad). We do this for our summer trips we take in the USA (we're also Canadians) and it works great. Ideally you use wi-fi to make calls since it is free in hotels, etc. 4G is excellent when on the road. We use the application MagicJack for the calls. Works very well. You need to have you iphone headset connected to the ipad and voilą!

The card is $30 for 3gigs. Good enough for the type of needs we have on the road.

Just food for thoughts.