I no longer use a landline, haven't done for years, but when I did I found the words "I don't have a computer" in the answering machine seemed to stop them.

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No "Hello, you have reached..." or anything else, just those words.
Many times when messages were checked only the sound of someone hanging up could be heard.
After a while word must have got around inside their circles I was a 'no sale' and eventually the nuisance calls stopped altogether.
Before doing that it got so bad I refused to answer the phone if it rang, letting everything go to the answering machine & turning it's volume down so I couldn't hear it, checking it at my leisure.

Registering my number with the Australian government's pathetic do not call register was a complete waste of time.
How that works is it requires the telemarketers to send them a list of who they want to call, so the register's staff can remove the registered numbers from it, then return it to the telemarketers with the registered numbers removed.
I'm sure you can imagine how well that worked...in fact I believe the calls got worse after registering my number...