Compared to other primates, human vocal behaviour is exceptionally flexible, generating somewhat of an evolutionary conundrum. What are the evolutionary origins of the human vocal flexibility in the primate lineage? We present data on Campbell's monkey vocal behaviour, which show that, although very subtle, important acoustic flexibility is present and determined by social factors. Previous work has shown that in this species, females frequently exchange particular vocalisations, the combined-harmonic calls, during social interactions. These calls can be classified into distinct subtypes and one of them, the CH6 subtype, is highly variable in its acoustic fine structure. Females differ in the number and types of CH6 variants they produce at any given time. Some variants can be produced by more than one female, indicating that females share variants. A female's pattern of variant production and variant sharing can change in response to important social events, even during her adult life. Here, we provide observational and experimental evidence to further explore the communicative function of this intriguing vocalisation system. First, our analyses showed that older individuals were more likely to elicit vocal responses from other group members than younger ones. Responses typically occurred within less than a second, suggesting that individuals answered each other's calls. In some cases, responding individuals matched their own call subtype to that of the initiating animal. Second, we conducted a playback experiment to investigate whether the CH6 variants used in vocal sharing were meaningful to recipients. We compared the group's response between currently and formerly used variants, i.e. calls produced by the same individuals at present or several years ago. We found that current, but not former, variants reliably triggered a natural exchange of calls in recipients. Instead, former variants caused a temporary decrease in calling rates, demonstrating that the monkeys discriminated these exceedingly subtle acoustic differences and that they were socially meaningful to them.