2600 says to forward them to voicemail and have your greeting start with the three-toned telecom information sound (bee baaaugh eeeeeep... at least in the US).

2600 says to forward them to voicemail and have your greeting start with the three-toned telecom information sound (bee baaaugh eeeeeep... at least in the US). Then they will likely take you out of their system as they identify that the number is not in service.