Abstract: We identified 104 trials including 111,653 participants that met the inclusion criteria. Participants were mostly adult smokers from the general population, but some studies included teenagers, pregnant women, and people with long-term or mental health conditions. Most trials (58.7%) were at high risk of bias, while 30.8% were at unclear risk, and only 11.5% were at low risk of bias for all domains assessed. Most studies (100/104) assessed proactive telephone counselling, as opposed to reactive forms. Among trials including smokers who contacted helplines (32,484 participants), quit rates were higher for smokers receiving multiple sessions of proactive counselling (risk ratio (RR) 1.38, 95% confidence interval (CI) 1.19 to 1.61; 14 trials, 32,484 participants; I2 = 72%) compared with a control condition providing self-help materials or brief counselling in a single call. Due to the substantial unexplained heterogeneity between studies, we downgraded the certainty of the evidence to moderate. In studies that recruited smokers who did not call a helpline, the provision of telephone counselling increased quit rates (RR 1.25, 95% CI 1.15 to 1.35; 65 trials, 41,233 participants; I2 = 52%). Due to the substantial unexplained heterogeneity between studies, we downgraded the certainty of the evidence to moderate. In subgroup analysis, we found no evidence that the effect of telephone counselling depended upon whether or not other interventions were provided (P = 0.21), no evidence that more intensive support was more effective than less intensive (P = 0.43), or that the effect of telephone support depended upon whether or not people were actively trying to quit smoking (P = 0.32). However, in meta-regression, telephone counselling was associated with greater effectiveness when provided as an adjunct to self-help written support (P < 0.01), or to a brief intervention from a health professional (P = 0.02); telephone counselling was less effective when provided as an adjunct to more intensive counselling. Further, telephone support was more effective for people who were motivated to try to quit smoking (P = 0.02). The findings from three additional trials of smokers who had not proactively called a helpline but were offered telephone counselling, found quit rates were higher in those offered three to five telephone calls compared to those offered just one call (RR 1.27, 95% CI 1.12 to 1.44; 2602 participants; I2 = 0%). There is moderate-certainty evidence that proactive telephone counselling aids smokers who seek help from quitlines, and moderate-certainty evidence that proactive telephone counselling increases quit rates in smokers in other settings. There is currently insufficient evidence to assess potential variations in effect from differences in the number of contacts, type or timing of telephone counselling, or when telephone counselling is provided as an adjunct to other smoking cessation therapies. Evidence was inconclusive on the effect of reactive telephone counselling, due to a limited number studies, which reflects the difficulty of studying this intervention.

Plain-language summary: We found 104 studies (including 111,653 participants) testing the effect of any type of telephone counselling. The participants were mostly adult smokers from the general population, but some studies also looked at teenagers, pregnant women, and people with long-term or mental health conditions. Some studies included participants who had called helplines that provide smoking counselling (quitlines). Other studies included people who had not called quitlines, but received calls from counsellors or other healthcare providers. Some studies provided telephone counselling alone, but many others provided telephone counselling along with minimal support such as self-help leaflets, or more active support such as face-to-face counselling, or with stop-smoking medication. The number of calls offered ranged from a single call to 12 calls. Some studies only recruited people trying to stop smoking, while others offered support even to those not actively trying to stop. Studies needed to compare groups whose participants had similar characteristics at the start of the study, to investigate whether the participants had stopped smoking for at least six months, and ideally would test whether people had quit with blood or urine tests. We judged few studies to be well designed and conducted. Most had at least one issue that could have affected the results. In people who had called helplines, providing additional telephone counselling increased their chances of stopping smoking from 7% to 10%. In people who had not called a helpline, but received telephone calls from counsellors or other healthcare providers, their chances of stopping smoking increased from 11% to 14%. In studies which directly compared more versus fewer calls, people who were offered more calls (three to five) tended to be more likely to quit than those who received only one call. Telephone counselling appears to increase the chances of stopping smoking, whether or not people are motivated to quit or are receiving other stop-smoking support. The overall certainty of the evidence was moderate, meaning that further research is likely to have an important impact on our conclusions.