The most popular brand is probably SpotMessenger.

http://international.findmespot.com/

You’ll be able to send back a canned text message or email, and with some models, you can compose a custom message on your iPhone, which will connect with the pocket-sized companion SpotMessenger via Bluetooth. In addition, these units provide a link so your family, friends, and/or home office can see your location on a Google-map style map. Amazing stuff. (You can literally see the 20-foot hut in which you are teaching!) These texts are about 10 cents each outgoing. But keep in mind, none of these units RECEIVE texts. So it’s only one-way info. These units start at about $75 and go up from there, depending on the features you want. On top of this, you’ll need to pay a $100/yr fee for the basic service, and more if you want extraction insurance. You can easily drop $225/yr. by the time you cover all the bells and whistles. With the extraction insurance, you can literally push a button that calls out the cavalry. They literally come and get you wherever you are in the world — and get you to safety.