if anyone is wondering. the Telcel sim card with data does work in the USA. We loaded 4 gb before we left Mexico. Currently in Pennsylvania and it is working great. Worked in Texas, Louisiana, Mississ...
"Telcel SLIM limite" is not seamless across the border.
According to them, 4 GB should work "sin frontera", i.e. any side of the border, and mostly it does See bottom line of table - but complaints are numerous, both on data and voice connection.
There is other plan - Optimo Sin Frontera - but I don't anybody who uses it, too expensive 2 pesos per 1 MB.