APRS has always been pointed out as a system that is more than a vehicle/station tracking system, it is in fact real-time communication, the tracking part is only one of a subset of functions the system has.
The system has the ability to send messages between users, normally this is done by virtue of APRS radio to radio, however it is possible to send a shorter than normal (you'll see why) SMS message to any mobile phone on the planet.
To send to a mobile phone, on any radio so capable of APRS or adapted to work on APRS with a TNC and client software you simply send a message from the radio to "SMSGTE" which must start with @<phone number> (where <phone number> is any mobile phone number starting with the country code though not the +) and any short message you like, as long as an igate has heard you recently it will gate the packet and send it on to SMSGTE which will in turn send it back to the mobile telephone network the number is associated with.
The system can allow the recipient to send a reply, though that reply cannot leave APRS-IS if the recipient was not heard by an igate, no one can reply to a message without knowing the correct syntax or the phone number the message from the radio originated from, so no chance of spam messages, particularly important considering my recent troubles with a very questionable individual from Australia though not enough to stop problems with Chance Callahan KD0MXN however he appears to have vanished from the Amateur radio scene for the moment and he could use standard APRS messages to harass me if he so desired as he does know my current callsign, though that alone is grounds for me to report him to the FCC for Part 97 violations.
So if you're out of range of the mobile networks and need to get a message to someone, use this, if you're up a hill the chance is are your message will get out to an igate (remember to beacon first so a reply can be sent from the recipient), I tested this and it works at least from the radio to the phone.
I have Carl 2E0EZT to thank for this information, as he has done a video on some of the APRS features available.
73 de 2E0EIJ