A crowd-sourced BBC survey of 3G coverage has found gaps across the UK, with people only able to get a signal for 75 percent of the time on average.
For the rest of the time, British smartphone users only got a 2G signal, the broadcaster said in publishing its results on Wednesday.
"The data shows that, overall, people are getting 3G about three-quarters of the time, but coverage is nothing like as uniform as the picture painted in the coverage map issued by the mobile operators," the BBC's Rory Cellan-Jones said.
The gaps, or 'notspots', extend across the UK, including in London and other key towns and cities, the BBC said. It published an interactive map that allows people to enter their postcode to check their coverage at street level.
Testers who volunteered for the survey downloaded an Epitro app to their Android devices that tracked the 3G coverage they were getting. The survey pulled in 44,600 participants, using more than 270 device models, in 42 million locations from the Shetlands to the Isles of Scilly, the BBC reported.
The results bear out the findings of Ofcom research published in May, which found "the availability of 2G, 3G or HSPA networks, and the performance delivered, vary significantly even within small geographic areas".
Operators broadly welcomed the BBC coverage map, though Three noted the results could reflect the number of people taking part rather than actual coverage. The broadcaster acknowledged this and also pointed out the gaps in coverage could be due to a single volunteer device losing its signal or reduced reception indoors.