These texts are sent via email using the carrier's email-to-text service. The program allows people to send texts to numbers under that carrier. It is intended for use by people who don't have phones with them (or at all) that want to contact someone with, for example, an AT&T phone. Since the sender isn't using a phone there is no return number, but the carrier assigns a reference number to the message (like 11010100001, 1101010002, etc.).
These texts are possibly two things. First, it could be a person who just typed in the wrong number and sent you a text message. Second, and more likely, it is a spammer who is testing the email-to-text service to see if they can use the service to spam mobile phone lists they have or just testing a program they wrote to spam mobile phones. I have began to receive spam from email-to-text service on my AT&T phone.
I know a lot of people are on the Do-Not-Call list, but I don't believe this situation applies. So until individual carriers address the problem, expect some texts from Canadian drug stores selling cheap Viagra.
If you "Pay per text" be sure not to hit "open" when your phone informs you that you got a new text. Open your text inbox and look at the preview or details of the text before you open it. Otherwise you will have to pay the $.10 or whatever.