When they call, tell them they reached an outside payphone. It might help get your number out of the system. They don't like payphones. Better yet, if you can cajole your telco to give you a number embedded within the bank of numbers it reserves for payphones (often the upper 9000s). I have two numbers at one place I live. The main number ,embedded among the payphone assignments, hardley ever gets called, whereas the second number LISTED AS A FAX LINE does get pestered by sleezy, slinky, slimy telemarketers because its a lower number. Best yet if you can get a number in the 9900s -- usually reserved for TELCO official use, sometimes for payphones. They are virtually never called.
Another ploy is to get an unlisted number just above a business or gov't office whose listed number ends in "000" -- say some big business place with xxx-xxx-7000, and having secondary lines scattered between 7001 and 7025. Sometimes there are unassigned numbers eg 7006 or 7011, which would generally not be dialed even by random or sequential number dialers ,as blocks of business numbers, payphones, gov't, and TELCO numbers are programmed out as worthless to telemarketers. This is also where some TELCOs put unlisted numbers for favored customers.