Today they called my cell again. I figured it was that kind of phone call because the caller ID number looks unfamiliar. I picked up the phone and whistled into the phone the "intercept" special information tone (which indicates the number has changed or disconnected). You hear this tone when you dial an incorrect number. I'm sure I didn't get the pitch perfectly, but after I whistled the tone four times, the unsolicited caller promptly hung up. I guess that was successful.
I'll have to see if the phone company complains to me for phreaking the phone's signal system, and if I really do stop getting the vehicle warranty call.