I do like the feaux-number idea, but I think it would be a hard trick to implement.
I've been getting these kinds of calls on my land line *forever*-- the same voices, both live person and automated, claiming that I needed to contact them immediately to keep from [insert doom and gloom about my car insurance/credit card/the Baldwin Brothers].
(Okay, I made up the part about the Baldwins)
I signed up for my state's Do Not Call list, but the list rules are a joke, since if these guys don't identify their company or their number, they can't legally be pursued.
But I was under the impression that cell phones were exempt from this kind of thing-- like fax machines *used* to be.
I don't know if it would be of any help in your situation, but awhile back I was getting text-message-spammed on my cell phone, and the originators were being listed as email addresses. Seeing as I didn't have email functionality on my cell, and I was paying 15 cents a pop for messages from people who I never gave my number, I called up my cell company and read them the Riot Act, telling them that if they didn't fix this, I'd not only be happy to take my business elsewhere, but also send a written complaints to the attorney general and department of commerce for my state, Consumer Reports, and anybody in the media who'd be willing to cover the story. I'm thinking somewhere in there I must've used some magic words, because I was transferred to someone who confirmed my info and informed me that it might take up to four weeks before it stopped.
Then just like that, I haven't received any spam since.
Now, while I'd like to believe that I'm all that, I don't think it was just my charming disposition that got results, so I'd consider giving your provider a call. And have your schpiel ready to rip.