Improved 2G and 3G signals are needed to increase growth and encourage visitors to return to the country, the Federation of Small Businesses (FSB) said.
Argyll and Bute, the Highlands and parts of Perthshire have the worst coverage according to the federation, which has responded to a Scottish Government consultation on plans to simplify the planning process for mobile operators to improve coverage.
The Government wants to make it easier to install phone masts and other equipment to help roll out the latest mobile and digital technology across the country.
Ofcom figures published in November 2012 showed 38% of the Highlands had no 2G signal, with the figure 39% in Argyll and Bute, 27% for Perthshire and 22% in Dumfries and Galloway.
Glasgow, Edinburgh and Aberdeen had virtually complete signal coverage.
In terms of 3G signal, 78% of Shetland had no coverage according to the figures, along with 44% of Moray, 40% of Perthshire and 70% of the Highlands. Most inner city areas had almost complete coverage.
FSB policy convenor Andy Willox said: "Too much of Scotland doesn't have adequate mobile phone coverage. Our members tell us that new technology will be vital to grow their business and our visitors tell us that they want to use their mobile devices if they're in the centre of Edinburgh or on the Isle of Mull.
"We're writing to the Scottish Government backing their proposals to amend the planning system and improve Scotland's levels of connectivity. But that can't be the end of the story.
"Businesses and their customers across Scotland want better coverage and we must see bold action to ensure that no part of the country is left behind. It is unacceptable that a quarter of Scotland doesn't even have a 2G signal."
"Official figures suggest that Scotland has the worst coverage amongst UK nations. We must get to grips with this problem before our businesses lose out to better connected competitors and visitors don't come to Scotland for fear of being cut off from the rest of the planet."
The public are being asked to put forward their views on the Government consultation, which would encourage developers to make better use of existing sites.
Planning Minister Derek Mackay said the plans are part of "Scotland's journey to becoming a world-leading digital nation by 2020".
He stressed that the proposals will encourage operators to make use of existing sites and do not relax the planning requirements for new masts.