Keep an old smoke alarm on your desk. The next time you get one of those telemarketers, put the phone over the alarm and press the button.
Totally understand the sentiment/frustration (we also get these calls despite the Do Not Call register!).

I too would love to do this.
However, using this tactic on the poor saps manning the phones is not really fair (I used to manage staff in a legitimate call centre for incoming calls only).
- The owners/managers of the Call Centre in some third-world country will not give a sh$t about the workers health or any compensation/sick leave
- The owners/managerswill not grow a 'conscience' if you do this
- You will damage the operator's hearing
- The operators are paid a pittance
- The operators are told to just follow their 'script'
- The operators time is micro-managed to the minute and they are under pressure to get results