Silence detection is easy for the phone spammers. There's a site that has bots to engage the spammer and get him/her to repeat themselves or explain further.

Silence detection is easy for the phone spammers. There's a site that has bots to engage the spammer and get him/her to repeat themselves or explain further. JollyRogerTelelephone.com isn't free, but they can email you an MP3 if you want evidence of how long a bot kept the spammer on the line.