I’m not sure what this has to do with 1-800 numbers (are you calling a 1-800 number? do you have a 1-800 number)? You shouldn’t be getting telemarketing calls on your 1-800 number, as you’re paying per call.
Get an answering machine and screen your calls. I used to have the voicemail thing, but I replaced it with a machine, so I could hear who was calling, and decide wether to answer or not.
I also have this tone on my machine, at the beginning of the message (see attachment). A lot of auto-dialers will mark a number as unreachable or disconnected when they hear those tones.
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