Calls to NHS Direct Wales should be free to patients and the current 0845 code scrapped, an AM has said.
The 24-hour non-emergency helpline costs 2p per minute but mobile firms charge extra with one woman claiming her 40-minute call cost her almost £20.
Plaid Cymru's Ynys Mon AM Rhun ap Iorwerth wants the Welsh government to make all calls free.
The Welsh government said work is being carried out to introduce a free line across Wales.
Carys Edwards, from Cardiff, called the service recently because her six-week-old daughter was unwell.
"My husband and I thought we had better phone NHS Direct and get some advice," she said.
"We were on the phone for around quarter-of-an-hour to 20 minutes and we ended up going to the out of hours doctor.
"It was around a month later we were looking at our mobile phone bill and realised that the call had cost me around £20 and I was understandably quite surprised and shocked.
"We thought we were making the conscientious choice of phoning NHS Direct rather than rushing up to A&E.
"One of the other mothers checked her bill and realised she had an £8 bill to NHS Direct."
Mr Iorwerth told BBC Radio Wales he could criticise the mobile phone providers but there was another solution closer to home.
"Take away this premium telephone number immediately," he said.
"Look for an 03 local charge number and then move as quickly as possible to introducing a Wales-wide free 111 number so people can ring to get the advice they need.
"They should know this is costing people, perhaps without money in their pockets, far too much to call this service at a time when people are actively being encouraged to seek alternative ways of contacting primary care in order to take the pressure off GPs and accident and emergency units - ie go to see a pharmacist or call a phone number like this.
A Welsh government spokesman said: "Calls to the NHS Direct Wales cost 2p a minute, it is the mobile provider's additional charges - which are set by them.
"Work is under way to introduce a national 111 line across Wales, which is a 24/7 free-to-call telephone service for non-emergency healthcare."