Message from a new inReach number

This has nothing (directly) to do with the originating number. Garmin uses SMS gateways to deliver iR to phone (SMS) traffic. The gateway depends on the location of the target phone. The gateway chooses the "from" number on the message that the recipient sees on the phone. This choice is essentially random. If you send several messages to the same number, each one might have a different "from" number.

In order to enable replies, the Garmin servers record the iR device ID, the outbound number selected by the gateway (which appears as the "from" number to the recipient), and the recipient's phone number. When the recipient replies, the reply goes to the gateway (which owns the "from" number). The gateway forwards the message to the Garmin server, along with the replying phone number. The server looks up the phone number pair, determines the iR device address, and sends the message on.

The information on the server has a limited lifetime on the order of several weeks. After that, replies no longer work. I normally "prime the pump" prior to each trip. I send a message to each person whom I want to be able to contact me on the trip. Each of them can simply reply to the most recent message from me. This ensures "fresh info" stored on the server.

Note that some gateways are not capable of handling the replies correctly. This is unusual, but does happen.

If you are sure that (a) the recipient has a fresh message from you AND (b) that the recipient is attempting to reply to the newest message, then you should open a support ticket. Because of the way some phones "thread" SMS messages, it's easy to become confused and reply to an "old" conversation - one for which the reply info on the server is no longer available. So be sure that's not the problem before opening the ticket.

Hi,

Thank you for your comprehensive reply and insight. I presumed there was some sort of lookup happening when somebody replied to an iR message - a bit like a router on a network.

The 'from' number appeared to change for all recipients when I changed supplier for my iR subscription. I went through a local supplier who set me up with a professional plan. I then went with a personal plan, directly with Garmin and that's when the 'from' numbers appeared to change. The new 'from' number for one recipient does not seem to allow them to reply. However, I have not ruled out the threading of the SMS conversation you mention and that's a good point.

One thing though, you suggest the 'from' number might change after several weeks but that's at odds with what I understand from Garmin from their web page below, where I have included the relevant text.

I suppose time will tell if the number changes and if it does, your 'prime the pump' method would be a good thing to do before setting off.

Thank you once again.

https://support.garmin.com/en-GB/?faq=o4sYSXP4LC8KhFNWJEVPI7

Replying to an inReach SMS (Text) Message

1. Open the message you received from the inReach user
   Note: If using a mobile device with a data connection, you can also visit the link included in the text message to view the inReach's location on a map
   
   
2. Reply the same way you would respond to any other text message

The inReach will receive the message the next time it connects to the Iridium Network.

As long as you keep the SMS (text) message you received, you can reply to it indefinitely to continue reaching that inReach user.

It's not that the from number changes after several weeks. It's that the server record of the "conversation" gets discarded. Not practical to retain that stuff forever.

Although I see it in the support article, I do not believe that the green highlighted part is true. Again, because it would require keeping that information on the server forever.

Note that it IS true with regard to using the link in SMS message to bring up the web page. THAT ability does persist forever.

I doubt that changing from an enterprise plan to a personal plan (by itself) would result in changing gateways. Maybe it's just coincidence that the from numbers changed at the same time. Or maybe something to do with the geographic location of the previous "supplier"?

This is an interesting academic discussion. I will be keen to see if people can still send me an SMS after a period of several weeks, without me sending them one in the meantime. I will test on my own phone in a month or so.

All (three) recipients saw iR messages from me change to a different 'from' number the very day my professional plan ended and my personal plan started. It's worth noting though, things didn't work reliably for about 48hrs after I changed. Some messages iR messages got lost in both directions as I was testing.

I suspect you're right about data being erased eventually but not much data is being held; just a link between a telephone number (the person sending the SMS) and the iR 'address' in whatever format that is.