I would think the easiest way to do this would be to ask the user for a phone number and network. You'd have to figure out what networks have what email address and then create the email address to send messages to their phone so it would be something like 2065551212@tmobile.net. Then you could add to the stored proc that gets called to check for trips that have just ended to have it look for trigger events that users have setup, this could be a big query. Then if you have sql mail setup you can send the email from sql or you can send it from the guage service, but you want to be careful not to slow down the guage updating.

There are probably other ways to accomplish the same thing, but this is an idea that would work, but could need some tweaking because it might affect other things.