What I've been doing for years is one of the pay-as-you-go Tmobile phones when in Penasco. You don't buy the monthly cards, though, get the MINTUES. Even a $25 card gets you 130 US minutes, or about 60 minutes and/or texts while in Mexico. You'd be surprised how little you'll need these, but it's nice to know it's there. If you plan on using it a lot, a $50 card has 400 US minutes, etc. If you've been doing this long enough, your minutes will sit on that phone for up to a year without usage and your local US number remains in tact. I keep the phone on the entire time while in Mexico and it just switches over to MovieStar (the Mexican cell service Tmobile works with) automatically as soon as I cross the border. Now keep in mind, this is a simple cell phone (costs as low as $30) with no data roaming, no online services.
I receive and can send texts back and forth to US numbers w/out dialing the extra country code, but when making an actual call I have to add the extra numbers. When people call from the states it doesn't always show who is trying to call, but at least I know something's up & I should probably check my email to see if there's an emergency. I do NOT get roaming fees for just leaving the phone on, it doesn't eat my minutes, only when texting or calling. I think the texts are like 60 cents each for sending or receiving and the calls are more like 80 cents a minute, still way cheaper than using a Mexican pay phone or, in the long run, even those pay cards, which almost always expire after 30 days, so you end up having to buy one each time you go down. ALSO, my prepaid Tmobile account gives me a US area code, so if someone needs to get a hold of me, I'm not asking them to use a weird international number with excessive fees. Additionally, many cell phone services won't let you text into Mexico period.
NOW, I also have a Tmobile smart phone, and I put that in airplane mode (so it won't roam) as I cross the border and keep the wifi on here and there. While in airplane mode, I can use the wifi signals that are at just about every bar and restaurant (and most hotels & condos) you may find yourself at in Penasco. I shut off the airplane mode once by mistake but caught it within a minute, didn't get any extra charges before the roaming kicked in. If you don't shut off the roaming on a data phone, you WILL get charged insane roaming rates unless you have some extra international plan, but most of the time the international plans only refer to the call minutes and not data roaming.
If you don't think you'll be near enough wifi signals, another option is getting the prepaid MEXICAN roaming data SIM cards for Tmobile/ATT smart phones, you just have to make sure your phone is "unlocked" with your cell phone provider before you cross the border, you can't do it once you leave the country.
