I have a Garmin In-reach that is useful for sending text messages via satellite. It connects to my phone via bluetooth and I use Garmin's app to type the messages. It uses the Iridium satellite constellation for service. You do need to see some sky and the more sky you can see, the sooner the message gets sent. They have a variety of service plans that you can choose from depending on how many messages you think you might be sending. You can buy an In-reach device from REI and return it if it doesn't do what you want. If the Garmin or Spot satellite devices won't do what you need, your only other option may be HF with an amateur radio license.