There is a working workaround to send InReach messages to AT&T phones - the email to text gateway.
"enter the recipient's 10-digit wireless number, followed by @mms.att.net. For example, [email protected]."
Source: https://www.att.com/support/article/wireless/KM1061254/
Both txt.att.net and mms.att.net work, but I found that the mms variant allows more characters and so fits all the text into one message, while the txt variant breaks the message into multiple messages.
