I got the same "mortgage" text. Online lookups tell you that it is an AT&T cell from Collierville, TN. But after talking with them, they say it is a pre-paid cell. She recommended to reply "STOP", like the text said. I am skeptical about doing so because it tells them they reached a real number. Looks like there should be a way to file a complaint against a number like that. SOME cell company has to be handling the calls. If not, prepaid cells should be outlawed.
After doing some checking, I found a place to reportt this kind of thing. Send the message AND add the sender's number to 7726, which spells SPAM. I first forwarded the message and received a text back asking for the sender's number, which is NOT sent when you forward a text. It's still in the experimental stages, but at least someone (AT&T) and others are trying to stop it.