Back in '92 when we first moved into our house, we got a new land line number and started getting collectors calls, we would explain the situation, the problem was some of them didn't believe us and continued calling.
The problem didn't go away entirely until we went wireless, some time in the 90s.
The only reason I have a landline is because my DSL provider demands it. We never use it. We get calls from time to time which usually aren’t for us. One was a bill collector. Because we didn’t use the landline, answering the phone was always optional. I looked forward to the bill collector calling. He was loads of fun to screw with. Finally I told him the truth and he sounded relieved and quit calling.
I had the same problem in 2006--and it went on for five years. The collectors believed me, but after a few months the uncollected debt would be resold (at a lower percentage of face value) to another outfit, and the calls and explanations would start all over.