On our last trip, we had a blowout in our 5th wheel on Mex 1 about 100 km north of La Paz. We didn't have a cell - but since we changed the tire ourselves, we didn't see the need for Green Angels. That said, we were asked several times by construction workers in the area if we were okay and needed any help.
Another time we got stuck in some sand at the beach. Passersby stopped and helped us and when they couldn't get us "unstuck" - gave my husband a ride to town to get help. Help came - all free of charge because it's just the Mexican way.
Having a cell phone where you can make the call is certainly a good thing - but the culture is such that people will help you and if all you need them to do is make the call for you, that's easy.
If you're really worried about it, get a cheap phone at TelCel for the journey once you cross the border.