A predictive dialer dials a list of telephone numbers and connects answered dials to people making calls, often referred to as agents. Predictive dialers use statistical algorithms to minimize the time that agents spend waiting between conversations, while minimizing the occurrence of someone answering when no agent is available.
When dialing numbers one at a time, there are two sources of delay. First, only some fraction of dials are answered. So, for example, if 1 out of 3 dials are answered, a predictive dialer might dial 3 lines every time an agent becomes available. Second, even dials that are answered take some time before being picked up. If it typically takes 10 seconds for someone to pick up, and conversations typically last 90 seconds, a predictive dialer might start dialing at 80 seconds.
Dialing one number at a time, only when an agent is available, typically keeps agents utilized for 40 minutes per hour (33% idle time). Predictive dialing can increase utilization to 57 minutes per hour (5% idle time).
In the UK, Ofcom requires that predictive dialers abandon less than 3% of answered calls on a daily basis. Ofcom also requires that if an agent is not available within 2 seconds the call is considered "abandoned" and an automated message is played. The automated message must identify the company making the call, the purpose of the call, a free phone or basic rate phone number to call back on and must not contain any form of marketing. A phone call to the return number must not be treated by the company as an opportunity to market, but to be removed from the calling list. In the UK "abandoned" calls must not be called back within 72 hours unless there is a dedicated agent available.
In the USA, if someone answers but no agent is available within 2 seconds of the person's greeting, Federal Communications Commission (FCC) regulations consider the call "abandoned" and require the dialer to play a recorded message. The FCC requires that predictive dialers abandon less than 3% of answered calls.
In Canada, the maximum abandon rate is 5%, and calls cannot be made to numbers registered with the National Do Not Call Registry, emergency or health care providers.
See also 
- Predictive Dialing for Outbound Telephone Call Centers, Douglas A. Samuelson, Interfaces, 29:5 September-October, 1999 (pp. 66-81).
- http://www.fcc.gov/cgb/policy/Telemarketing-Rules.pdf PART 64 – MISCELLANEOUS RULES RELATING TO COMMON CARRIERS.
- http://www.crtc.gc.ca/eng/trules-reglest.htm Canadian Radio-television and Telecommunications Commission Unsolicited Telecommunications Rules - Part III