The present paper reports an investigation of the potential impact of introducing common-channel signalling (CCS) into the current telephone network. This technology would have the effect of greatly diminishing post-dialling delay (PDD). As such, its main benefits would be obtained by introducing it into the toll network, in which PDDs are much longer than in the local network. The issues examined concerned potential ‘contrast’ effects, in that reducing PDDs in the toll network may cause callers to be less patient with normal PDDs in the local network. Three laboratory studies were undertaken to evaluate caller impatience and abandonment under (1) the current system, (2) the current local system with a simulated new toll system and (3) the current toll system with a simulated new local system. Ratings of impatience and abandonment increased on local calls when the new technology was implemented on the toll network, but not vice versa. The explanation offered is based on a ‘cognitive’ contrast effect resulting from callers' expectations that toll PDDs should always be longer than local PDDs. The implications of this effect for caller behaviour with the introduction of CCS are discussed. Any negative effects on local call behaviour are outweighed by the much shorter PDDs on the toll network and should be counteracted by the gradual introduction of CCS.