Introducing GP consultations over the phone, rather than face-to-face appointments, does not reduce pressure in busy surgeries, a study shows.
The analysis of 42 practices, in the Lancet, found telephoned patients needed more contact with their GP.
The report's authors said phone consultations had a role, but were "no silver bullet".
The British Medical Association (BMA) said GPs were "under real pressure" from soaring patient demand.
It was hoped that using telephone consultations would relieve some of the burden.
An average surgery deals with 20 patients who need an appointment on the day.
Researchers at the University of Exeter Medical School investigated the impact of having a GP or nurse call those patients back instead of them coming into the surgery.
Rather than easing workloads, the number of patients being dealt with by practices increased by 33% when a doctor called back, and by 48% when a nurse was on the other end of the phone.
The study showed that telephone conversations led to more people needing a second consultation.
If the first meeting was in person, 50% of patients needed a second visit, but 75% of GP and 88% of nurse calls required a later visit to the doctor.
Lead researcher Prof John Campbell told the BBC: "This is not the silver bullet to dealing with workload.
"Introducing it in some settings will be very useful, but it needs to be introduced with caution."
He said phone calls were still a valuable and good service, especially for people who could not get away from work.
But equally they could harm the relationship with patients who like to come into the surgery.
Dr Richard Vautrey, the deputy chair of the BMA's GP committee, said: "A key problem is that general practice as a whole is under real pressure from soaring patient demand and falling funding, which often means that practices don't have enough GPs or nurses to operate these services in addition to managing their current workload."
He added that phone services increased demand from people who would not have come to an appointment, or would have managed their condition at home.
He said it was important to get the balance right and ensure face-to-face consultations were not completely replaced, as "there are many conditions that can only be safely treated by direct examination in the GP practice".
A spokesperson for the Department of Health said: "Most patients can get appointments and we're offering 7.5 million more people email, Skype and evening and weekend slots.
"GPs know what works best for their patients so can tailor their services as they see fit and based on what patients need."