Hello there @Shagreg!
I am happy to hear you are taking advantage of monitoring your online bill. We always encourage staying atop of charges displayed during your monthly cycle. I will be pleased to share additional details.
The digits would really depend on which number texted you the particular message showing on your bill. If the incoming number looks like a short code like what you listed, it may rather be an auto service. For example, services or businesses you sign up for coupons and notifications via text, their “phone number “can look like a short code. I suggest monitoring your incoming text messages throughout the month to determine which service messaged you.
Please let us know if you need any additional assistance. Have an awesome day!
Shenoah, AT&T Community Specialist