It’s likely a robocaller, in which case, send them to a recording of the SIT intercept tone.
This thing: en.m.wikipedia.org/wiki/File:IC_SIT.ogg

It needs to be the first thing the machine hears, so might not work as a voicemail welcome message if your provider has their intro before your message.

It might take a few calls/days before you’re purged from their multiple instances of robocallers.