But Mr Lewis said rescue teams were often left to work out themselves where an address might be in the local area.
Because the details often come to them via emergency services, rather than directly from the person needing help, it is not always possible to phone them back to confirm addresses.
"That in itself can be frustrating," said Mr Lewis.
There are alternative systems such as OS Locate, a free app provided by Ordnance Survey, that allows people to locate themselves by GPS latitude and longitude co-ordinates as well as altitude, even without a mobile signal.
Sarloc and Phone Finder are tools that have been developed by mountain rescue team members, and these are used whenever possible to obtain an accurate location.
"While we welcome anything that could contribute to safety, we're wary of adding to people's reliance on their mobile phones." added Mr Lewis.
MREW is keen to encourage anyone planning a day in the outdoors to carry a paper map and a compass - and know how to use them - as well as a torch or head torch to use in the dark. They also advise registering your mobile for the text 999 service. On a windy or poor signal day, this can really help.
W3W has been in the spotlight in recent weeks following research by security consultant Andrew Tierney, from Pen Test Partners.
He found that the algorithm behind W3W often gave similar-sounding words and plural versions of words for locations in close proximity, which could cause confusion.
So, for example, circle.goal.leader and circle.goals.leader are less than 1.2 miles (2km) apart along the River Thames.
In response to the latest finding, Mr Tierney told the BBC: "This is not an algorithm issue. Something like 73% of What3Words addresses contain a word that can be changed just by adding or removing a letter."
Combined with mispronunciation and the use of plurals, there was a big margin of error, he said.
"I estimate that about one in 30 squares would be confused, and so these mistakes are going to happen frequently," he said.
"My gut feeling is that a much better system could be designed, using a simpler dictionary, and maybe using four words instead of three."