They don't work anymore... Most of these are all SIP based now days, so they are just looking at the SIP responses and don't even listen for tones. We had something similar setup for a customer using the Inter-tel Axxess Voicemail system at one time on a line that got LOTS of telemarketer robo calls, it worked really well initially in the early 2000's, but gradually quit working as years went on and around 2012 (?) we upgraded that system to a 5000 and migrated it over to that and it still seemed to work sometimes. A few years ago they switched to SIP trunks, seemed to have no effect after that and we turned it off a year or so later.