The day we leave Mexico in the spring we put 100 pesos on each of our phones, this time is good for two months (60 days). We understand that after that we have 180 days to re-activate the phones simply by going into the first OXXO or what ever and putting another 100 pesos on the phones and are good to go.
We leave near the end of March and cross back into Mexico in early November and have maintained the same phone number for about 6 years doing it that way. Of course, you lose the 100 pesos you put on in the spring (and any remaining balance you had) but to us it is worth it in order to keep the same numbers.
We don't do banda ancha so can't answer to that.