The Telephone Consumer Protection Act (TCPA) provides a (deceptively?) simple definition of an Automatic Telephone Dialing System (ATDS):

The term “automatic telephone dialing system” means equipment which has the capacit - equipment which has the capacity -

(A) to store or produce telephone numbers to be called, using a random or sequential number generator; and

(B) to dial such numbers.

When Congress enacted the TCPA, the definition of an ATDS was specifically intended to regulate equipment that dialed random telephone numbers or large sequential blocks of telephone numbers, so as to protect consumers who had unlisted telephone numbers. S. REP. 102-178, at 1-2 (“Having an unlisted number does not prevent those telemarketers.”); H.R. REP. 102-317, at 10 (discussing the ability of an ATDS to “dial sequential blocks of telephone numbers”). The Federal Communications Commission, which has rulemaking authority relating to the TCPA, issued orders in 1992 and 1995 endorsing this interpretation: dialing equipment must be capable of generating random or sequential blocks of telephone numbers to be an ATDS.