A study of telephone quitlines shows that they double the chance of quitting smoking successfully.
Clinical trials conducted in the early 1990s showed that telephone counselling (so-called 'quitlines') help people quit smoking for good.

They are helpful in reaching many people who mightn't be able to attend a clinic.

But how are the quitlines doing in the 'real world'? Is the benefit sustained outside of the clinical trial context? At present, 33 states in the US have quitlines and to assess their effectiveness, researchers in California looked at what was happening in their own state. They carried out a study that compared the effect of using the quitline with using self-help without telephone counselling. It turned out that over a year of study, those who used the quitline were twice as likely to quit smoking successfully compared to those who did not use the phone.

Source

New England Journal of Medicine 3rd October 2002