The NHS 111 service is dangerously understaffed with up to 75% of calls going unanswered in some areas, it has been reported.
A single nurse was left to cover an area containing 2.3 million people at one centre, a whistle-blower told the Daily Mail.
Call handler Irsah Tahir said workers at her centre were swamped by calls from across Derbyshire, Leicestershire, Nottinghamshire and Northamptonshire.
During the busiest times in the East Midlands up to 75% of 111 calls went unanswered, and around half a million people across the country were unable to get through to anyone, according to figures in the Mail.
The Royal College of Nursing warned the non-emergency number will be "completely overwhelmed" this winter if ministers fail to act.
Some patients were forced to wait up to 11 hours for a nurse to call them back and the service failed to meet call targets seven months in a row, according to a Mail investigation.
"I feel the service is completely unsafe," Miss Tahir told the paper.
"I find it terrifying, it's just chaos.
"The whole time I was there I felt like it was just a matter of time before something happened.
"We were always short-staffed. There was never enough nurses.
"I'm not medically trained and I just don't think I was equipped to make those decisions."
She also claimed there were not always nurses on duty to supervise staff with no clinical training.
Staff were sent a series of emails begging them to do overtime or cancel their holidays because of staff shortages, the Mail reported.
An NHS England spokesman admitted there are have been issues and "regional variations" with the service.
"Across the country NHS 111 continues to offer a prompt and safe service, with growing numbers of patients phoning 111 when they might otherwise have gone directly to A&E," he said.
"Nine out of 10 calls made to the service are answered within 60 seconds, and latest surveys show that 90% of callers are satisfied with the service they receive.
"While 111 is performing well nationally, there are regional variations.
"Where there have been issues, we have been working closely with local CCGs to ensure these are addressed.
"If performance is consistently below standard, this will be taken into account when local 111 contracts are reviewed early next year."
Labour MPs Keith Vaz and Liz Kendall and Conservative MP Andrew Bridgen are demanding an immediate investigation into the service.
Health minister Ben Gummer told the Daily Mail: "NHS 111 is a vital service helping people to get medical advice and support, with over seven million calls in the first six months of this year alone, significant increases on what NHS Direct dealt with.
"90% of callers reported they were happy with the service, which is encouraging, but ministers will look closely at evidence of any problems to ensure that all patients throughout the country are getting the consistently high standards they deserve."
The 111 non-emergency NHS number was set up to offer help to patients and direct them to the best medical care.