It!
This won't help much now, but I will be able to report soon. In a few weeks I will be taking a trip (driving) to MX from PHX. I plan on leaving my phone on, with International Data Roaming and International Voice enabled. I have already ensured several times that the International Value Roaming plan is indeed active on all my Sprint devices. Although most of my trip will be in Movistar coverage, there are a few gaps, which is what I am most curious about. I will leave my phone on the entire time, just to test. Again, it will be a few weeks before I take my vacation but, I will report back.