[3] In their earlier forms, diallers would be proprietary standalone systems that connected directly to a private branch exchange or even to the public switched telephone network.
[4] In its most primitive "preview" form, an auto-dialler operates by first presenting contact details to the call centre agent on a computer display, who then initiates the call with a mouse gesture, a keyboard press, or some other human input device action.
[5] This was such a problem in some jurisdictions in the early years of the 21st century that government regulators imposed rules upon companies that used auto-diallers; Ofcom in the United Kingdom, for instance, imposing a rule that an auto-dialler had to at minimum play some sort of recorded message identifying the calling party to the called party within 2 seconds of connection, and constitute no more than 3% of the total outbound call volume in a 24-hour period, or the company in charge would pay fines of anywhere between £50,000 (equivalent to £80,975 in 2023) and £2,000,000 (equivalent to £3,239,000 in 2023).
[8] After pressure from telemarketing companies, who claimed that this was simply not achievable with the technology of the time, Ofcom extended the permitted period of silence.
[7] [9] Further improvements are thus the "predictive" dialler, which uses heuristics, and the "progressive" dialler, which directly keeps track of agent availability, and does not make further outbound calls where no agent would be available to handle the call when the callee answers.