In fact many times my phone does not even roam in TJ and uses US Cell towers.
Originally Posted by Fizley
That was true about fifteen years ago, when the US was still pre-3G. We used to carry two cell phones (one 3G Mexican and one 2G US) and the US phones only worked where there was line-of-sight to the cell towers in San Ysidro.
Since then, like maybe even ten years back, the laws here changed such that foreign entities may purchase shares in Mexican telephony, banking, and retail gasoline and consequently cross-border consumerism is almost seamless now. For example, I now carry only one cell phone in the Mexican AT&T network and can use it throughout North America. Best to check with your own carrier to see what their terms are, though.
As for banking, B of A, Wells Fargo, CitiBank, and Chase all bought shares in one Mexican bank or another partly to encourage Mexican workers in the US to send cash to their families in Mexico. However, they failed to update their cards to include RFID chips, a requirement in Mexico and Europe, so many US cards continue to be rejected at ATMs and points of sale down here. Most US banks will issue you a chipped card if you ask them, something you would want to do sooner rather than later.