The Onstar is active for Emergency only for most National vehicles. It's not active for Navigation. So if you have an accident you should be protected, but if in doubt ask a Manager at your rental location.

If you are worried about your Canadian roaming issues, perhaps you could buy a phone in the states that's the type sold at most discount stores like 7-11 or Target or Wal-Mart...that's a Pay-as-You-Go style phone. Buy the phone for cheap and loads the mins as you need.

Or if your phone allows you to buy a phone chip for other phone systems, do that.

Or borrow a phone from a friend that allows that.