We had the same problem.
We bought an answering machine and put the message on it we were not interested in any sales pitch. Turned the ringer and volume down as low as we could and put it in the spare bedroom. Left it that way until the 30 days were up and we were on the Do Not Call list. Friends could still leave a message but we didn't have to answer the phone and put up with
.
That was a while ago. Don't even know if they make answering machines like that anymore.