>Currently I have two landlines for my business. One line is practically never used. The other line (main line) used 90% of the time by telemarketing people. Maybe 10% is used by customers.

I keep the landline for the dozen people who are still calling from time to time and are too old to switch to mobile.

For as long as they live, I'll keep the landline, also as a reserve for the days when my Pine64 acts up and doesn't receive calls (something in the schema - works on quiet, doesn't on full or silent). My piece of wisdom that I delivered to some clinic's telemarketer today: „What, that I should allow you to put your instruments on me? You aren't able to remember that you shouldn't ever call this number again, in six tries you still didn't get me off your list, I wouldn't charge you to keep two drawn geese!“.