Upgrading your plan may be a good way to avoid overage charges, but Sprint might have another option: WiFi calling. 

Another carrier (potentially) allowing their users to make calls over WiFi. The value prop makes sense (“This will allow you to make calls when your cellular reception is poor”), but it is also a big defensive move to offer their own alternatives to 3rd-party OTT apps like Line.

Sprint might launch free WiFi calling for select Android devices