How many items (email or phone number) are allowed for Preset messages?
I've never actually tested this. For preset messages, the recipient list is set up and stored on the Garmin servers. The recipient list is not used to address the message form the device itself. Rather, the server "expands" the recipient list when you send a preset.
Basically, the answer is "however many explore.garmin.com will let you set up".
haha - the same, maximum is 4 for one of my Presets. I am trying to be reasonable user for any system, and even Presets do not cost me extra money, I dislike idea simple overload (ok, or just unreasonable create extra load) for Iridium - inReach - Garmin infrastructure with my experiments. Actually system is designed to help and save.
Trust me, one-off messages aren't overloading their systems - tracking points use WAY more of their system resources, since many IR devices are constantly transmitting coordinate data and/or pinging the Iridium network to check for new messages every 10 minutes or less. So while I might send 2 or 3 preset messages during a 4 hour hike, the IR is also transmitting 24 tracking points to Iridium plus an unknown number of message check pings during that time. Their system is designed to handle it just fine, I don't think there is any concern about overloading it.
Also it's important to know that sending a preset message, regardless of the number of recipients it has, only transmits 1 message to the iridium network. Iridium then forwards it to Garmin, and Garmin then forwards it to the recipient list via the normal cellular/internet land networks. It doesn't use any more of Garmin's network bandwidth than it normally takes to send SMS or email messages, which is very little.
The Iridium network was designed to handle voice (telephone) communication; short text-only messages use comparatively VERY little of their bandwidth compared to the phone calls they are also handling.
Preset messages are the least resource-intensive message you can send. The preset is text is not actually transmitted - only a code which identifies the message. Recipients, of course, are not transmitted either since they are filled in by the server.
That said, a preset does require slightly more data than a track point.
When you are sending track points, a message check is performed as each track point is sent. However, there is no additional data cost involved in that check. The check and the response (yes message waiting, or no message waiting) piggybacks on the track point and the acknowledgement from the Iridium network. There is additional cost to fetch the message(s) if there are messages waiting.
Most iR devices automatically do an active mail check once an hour, independent of other activity. That one does incur a data cost, but it is small unless there is a message waiting.
Yes, text messages use less bandwidth than voice. However, the service is priced accordingly. This is why the message allotment varies by plan. And why inReach service is cheaper than satellite voice.