Most just send out gps coordinates (NMEA sequece) or keep a data log. Some will sent gps coordinates via remote call in or by pressing a button in the transponder. You will need GSM ie mobile connectivity for most of them to work.
Reliability? That is a million dollar question
. Even space shuttle can crash
. If you want reliable tested units with supported warranty, Use the Garmin Rhino. BIG and work both ways but range is an issue. Next best is the dog tracking unit, slightly longer distance but one way. For the price you can get from lelong at subUS$50/-... no harm testing