There is a danish guy who started answering all business spam calls with "we do not use X" here - so they would call up and ask for whomever was in charge of lighting and he would say "we do not use lighting here", then spin a story about how they just used very bright monitors etc.
You might be able to have some fun with them at least - or just abuse the heck out of them (unfortunately telemarketers seem pretty blasee about it).
As for your private line, if you don't recognize the number just don't answer it? My cellphone blocks all calls during the night (with an exception for families) but I think it can block all numbers not in my contacts.
My co-tenant pays various utility bills so I already respond that I don't pay an electricity bill, water bill, etc - that usually leaves callers quite puzzled.
You might be able to have some fun with them at least - or just abuse the heck out of them (unfortunately telemarketers seem pretty blasee about it).
As for your private line, if you don't recognize the number just don't answer it? My cellphone blocks all calls during the night (with an exception for families) but I think it can block all numbers not in my contacts.