A text mail subscriber is someone who uses the internet to make phone calls, send text messages, or send emails. You can't call them because they don't accept phone calls, but you can sent a voicemail, which will be translated to plain text and sent to them.
See Also this post:- https://assistsuite.puzl.com/_news/What-is-Text-mail-Subscriber-Everything-you-Should-Know%253F/298178