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Many animals produce vocal signals during agonistic interactions that convey their ability and motivation to escalate the conflict. Often, such disputes follow ritualized, sequential phases with increasing aggression, favouring the assessment of individuals' chances of winning. This allows for individuals to withdraw before getting involved in a serious fight. Birds are well known for the variety of information encoded in their vocalizations, sometimes including specific features that signal aggression. A particular case may be so-called ‘tret calls’ produced by chiffchaffs, Phylloscopus collybita, a song affix for which the communicative role is not well understood. Here, we describe the acoustic structure and use of tret calls during simulated conflicts using playback. Our results show that tret calls are a low-amplitude vocalization and that birds increased tret calling after playback stimulation, suggesting a role as an aggressive signal similar to soft song in other species. However, we did not find that the tret call rate predicted subsequent aggressive behaviour. Our analysis of song variation in response to playback suggested that birds respond to a simulated vocal competitor in two phases: a highly aggressive phase where syllable rate goes up, which is predictive of physical attack, while simultaneously tret call rate goes down, and a low-aggression phase, in which tret call rate goes up, while syllable rate returns to baseline levels. These stereotypic fluctuations in two acoustic features may be part of an advanced communication system to moderate aggression. Finally, our results show a seasonal increase in tret calling after birds arrive at their breeding grounds, which is in line with a role in territorial aggression. We also found a nonsignificant trend for more tret calls with increasing ambient noise levels, which may be due to increased stress and aggression under noisy conditions or reflect a masking avoidance strategy.