Health watchdogs have launched an inspection of a NHS 111 service where teenagers without the right medical training were said to have been drafted in to answer urgent calls.
It comes after claims medics at the helpline in the South West were so overworked that they fell asleep exhausted at their desks.
It is the same service where a call handler failed to recognise that one-year-old William Mead, who died following a string of NHS failings, had sepsis caused by an underlying chest infection.
Responding to reports in the Daily Mail, the Care Quality Commission said it would probe the allegations raised by a former call handler-turned whistleblower.
Deputy chief inspector Ruth Rankine said: "These allegations are unacceptable. We take them extremely seriously and are planning to carry out an early inspection to investigate.
"We have also been working closely with commissioners and local partners to make sure patients are safe.
"It is critical that patients using urgent and emergency care are assured it is safe, effective, caring, responsive and well-led."
A spokeswoman from the CQC said staff at the service covering Devon, Dorset and Cornwall would be interviewed as part of the process.
The watchdog's announcement comes after Sarah Hayes, a former senior call adviser for the helpline in the South West, said she had to speak out as she felt the service was "unsafe".
According to reports in Tuesday's Daily Mail a number of 17-year-olds were brought into the service last year, allegedly to meet call answering targets.
There was no suggestion of any wrongdoing by the teenagers, who were said to be hardworking.
The teenagers, who were said to have only been authorised to take patients' names and details or offer basic advice on where to find a chemist or health service, reportedly answered urgent calls.
Ken Wenman, chief executive of South Western Ambulance Service NHS Foundation Trust, insisted that patient care and safety were "top priorities" for the organisation and its own investigation had been launched.