Check the number on popular tracking apps and websites; Mostly popular websites charge a fee to look up a text mail subscriber number.
If you contact the appropriate authorities, you can get a court order requiring the text messaging service provider to disclose the user's identity.
With the help of Trapcall to find out who is calling and to add callers to a blacklist.
Follow above given steps to track a text mail subscriber.
See also this post:- https://www.completefoods.co/diy/recipes/what-is-a-text-mail-subscriber
https://www.onfeetnation.com/profiles/blogs/how-to-track-a-text-mail-subscriber