TelCo code for number not in service?

I assume you are in the US because this is illeagle in Canada the CRTC regulates non-solicited business to busines calls if your request for list removal is not respected, the caller may lose his business licence and receive hefty fines. To date, I have yet to receive a single telemarketing call on my business line in 7 years.

WHY Telezapper doesn't work on digital lines:
Digital lines, although they assimilate DTMF tones, actually do not rely on them for connection. The tones, similar toyou cell phone's dial tones, are just there so you can hear the number being dialled they don't actually perform tip or ring signalling because the DTMF is converted to Binary at the PBX or Key Service Unit in the company's telephone room.

An analog line carries DTMF tones through the line which reach the CO and get rerouted to the analog termination point (your phone at home).

A digital line DOESN'T carry ANY DTMF singalling, the call and DTMF tones terminate at YOUR PBX and then are converted into binary code for digital connection via you T-1,T-3, Centrex, ISDN BRI/PRI line.

The Telezapper sees the incoming DTMF Tone (through you analog line) and simply scrambles the DTMF tones to return the connection as invalid. Although they are said to SQUEAL in the users ear, this is just a wives tale, they actually redial DTMF codes at high speed, similar to having the called party just hit buttons quickly causing a beep beep tone... in your ear.