I have a Spot Gen 3. It sends pre-defined messages and does tracking as well.
You can make the OK, Custom, Assist buttons say whatever you want, but you have to configure it on their website before you go.
I once set my OK button to "I'm OK. 3x in 1hr means bear down, come help" ... that came in handy. Dumped a big bear just as the heat was kicking in. Hit OK on the spot each time it had finished the prior message while I worked on the bear. Wife and brother showed up just in time for the packout
Careful adding the phone numbers / emails though. Turns out for years I was sending "I love you and miss you like crazy" (custom button) to some random dude, along with my GPS coordinates. Never realized it because my wife was getting the message via email and didn't know it was supposed to go through txt too.
It does take 15-20 mins for a message to go through. It seems mostly reliable. Out of a couple hundred messages sent, I think we had 3 that did not go through, but did not show up as a failed send (red light) on the unit. That said, if I ever had to use the SOS button, it's enough I'd wonder if the message went out... though I "think" (look it up to be sure) that SOS keeps transmitting while a subsequent SOS press cancels it.
It's good enough for me anyway. A bit pricy though. I think my subscription was around $230 USD this year. There are probably better options for the money. I reach maybe?