hmmm one way roun it i see is having some kind of internal prog in phone that before it rings/transfers to voicemail it requests of caller that they press a certain number or combination of numbers on their handset like

"Please enter 1, 7, 9, #"

if code is not entered as instructed call is dropped so person doesn't recieve nuisance calls from SPIT nor can it leave voicemail.

would mean people with older phones cant get through (if they dont have touchtone) but small price to pay

v_Ln