Keeps calling but nobody ever on the other side

No message left - call out of Florida - and coincidentally I just received a string of text messages from "402-30" that appear to be spam/junk solicitations for $9.99 mo. subscription.  Of course, I only read what popped up, I did NOT open the texts.  The texts say to reply with "stop" to cancel, but I'm afraid to even do that.  Does anyone know what this is about?

Unwanted call to cell, no one on line

Called again today --- ignored call, no message left

Called twice but hung up as soon as I answered. When I call back I get a message saying the number is disconnected or no longer in service. I tried to text and got a message saying the number is a landline. I started getting calls just after posting an ad on Denver craigslist.