1. A method for evaluating an effectiveness of a prompt sequence played by an interactive voice response system, the prompt sequence containing a plurality of separate phrases, the method comprising: by an interactive voice response computer, playing an initial prompt sequence to a plurality of callers; by the interactive voice response computer, receiving indications of a plurality of caller events during playings of the initial prompt sequence, the indications including a plurality of timestamps of the plurality of caller events; determining that at least one caller event of the plurality of caller events is a caller hang-up of a misdialed call, based on a timestamp falling immediately after initiating the playing of the initial prompt sequence; mapping the plurality of timestamps of the plurality of caller events to a particular phrase of the plurality of separate phrases, wherein the at least one caller event is removed from the mapping as a caller hang-up; determining that the particular phrase is ineffective based on the mapping of the plurality of timestamps of the plurality of callers to the particular phrase; in response to determining that the particular phrase is ineffective, creating an alternate prompt sequence containing an alternate phrase to the particular phrase; selecting and playing for each of a plurality of callers a prompt sequence selected from the initial prompt sequence and the alternate prompt sequence, the selecting being according to a pattern of playings without gathering a complete data set on one of the prompt sequences before gathering data on the other, thereby avoiding systematic errors due to a time of day of a call; receiving indications of a plurality of caller events during playings of the prompt sequences, the indications including a plurality of timestamps of the plurality of caller events; automatically comparing an effectiveness of the particular phrase with an effectiveness of the alternate phrase based on mappings of the plurality of timestamps of the plurality of callers to the particular phrase and to the alternate phrase; and automatically replacing the particular phrase with the alternative phrase if the alternative phrase is more effective.