How does one stop AT&T Salesmen calling 3 or 4 times a day every day

As it is a business phones, and I  deem it necessary to answer the phone manually; yet because for the last 26 days I have turned them down on installing VOIP, they call   3 or 4 a  times a day,  and do not answer as they probably recognize my voice from the previous 25 days saying  No loudly to them.  I have told them take me off their list, but I imagine AT&T Contracts these out 4 or 5 different 3rd-Party Companies.

Hopefully, the VOIP drive will come to an end but it is a nuisance.