They are the partial "not-spots", where there might be signal from one or two operators but not all four major ones (O2, EE, Three, and Vodafone). If you're on the wrong network, you have no coverage at all in these areas.
It's something Culture Secretary Sajid Javid is fighting to change. The government minister thinks that signal bars disappearing while rambling in the countryside, or commuting on a suburban train, is unacceptable - and even damaging to the economy.
In a report today he says mobile phones are a "central part of our lives" and that everyone should have a full choice of providers, writing that the worry of losing connectivity when visiting another part of the country should be eradicated.
Javid's plans centre around a "national roaming" framework, where phones are able to pick up another network when their default service goes - similar to what happens when travelling abroad.
The proposal also includes a "coverage obligation" for mobile networks that would require them to cover an improved percentage of the
But not everyone is a fan of Javid's idea. And phone companies such as Vodafone and EE are being particularly vocal. They don't want to be required to cover the cost of carrying non-customers. In the US, by contrast, mobile operators are happy to carry non-subscribers' calls - because they charge them extra for the privilege. The Javid plan, however, appears to require companies to provide everyone with coverage for no extra cost.
It would be great for customers, and bad for companies in other words. (Those companies have spent years touting their coverage as superior to their rivals - this plan would render all that irrelevant.)
The Inquirer says Vodafone argues coverage quality would actually be made worse by Javid's plan, claiming a "higher risk of dropped calls" and the likelihood of poorer battery life.
Rival EE says the concept is "flawed", and tells The Inquirer: "This will deteriorate network reliability for tens of millions across the UK, plus it also risks prices rising, which customers understandably won't tolerate."
It's not just the phone companies, though. Research analyst Matthew Howett of Ovum tells the BBC the plan is a "messy solution"; in another blow, The Times reports a leaked letter in which Home Secretary Theresa May suggests allowing cross-network coverage might "compromise efforts to track criminals and terrorists."
May has risked Cabinet in-fighting by wading into Javid's project, expressing her fears that the network improvements would risk the work of intelligence agencies.
Javid's "national roaming" works by allowing competing networks to use each others' masts, while giving virtual providers like Virgin and Tesco access to the four main players' resources.
Now, a consultation into 2G coverage is ongoing. Those in the mobile industry, as well as other businesses and the public, have until November 26 to respond.