When a call is received from an unrecognised caller, trueCall answers the call and plays the announcement :-

“If you’re family, a friend or an invited caller please press 5, if you’re a cold caller please hang up and don’t call again”.

If the caller presses ‘5’ your phone will ring. 

• silent calls and recorded message can't press 5 so can’t get through
• many call centre systems use Answering Machine Detection technology – this hears the announcement, thinks that it’s reached your voicemail so hangs up
• most telemarketers terminate the call when they hear the announcement – they know you are very unlikely to accept their call, so it makes sense for them to move on

In projects run by the police and local authorities this simple process blocks 95% of unwanted calls.