For mobile phone networks to operate, they need enough radio spectrum and the base station infrastructure to handle it: the towers and masts.

Radio waves from the phone are sent to the nearest base station, where they are connected nationally, transferred to the call recipient's network and sent from their nearest base station to their phone.

Base stations have a limited geographic reach and finite capacity, and are usually sited where there is strongest customer demand.

Connecting base stations is usually much more expensive in rural locations, with many hundreds of metres of trenches and cabling needed – something which can put operators off.

The government's Mobile Infrastructure Project (MIP) has made £150m available to improve mobile coverage and quality where the commercial case is not strong enough.

Where a network's frequency is on the radio spectrum also has a major impact on the quality of the signal for phone users. The lower the frequency, the further it can travel, and the more penetrative it is.

Vodafone and O2, as two of the more established networks, operate at a frequency of 900MHz, while newer networks have had to find their place at the top end of the spectrum.

The network 3, established in 2003, operates at 2100MHz, though it has recently acquired mid-range frequencies at 1800MHz and low-range at 800MHz, which it expects to significantly improve its call coverage. There are currently more than 80 million mobile handsets registered in the UK, and data-hungry smartphones have upped the demand on networks once more.

Data signals are dependent on being close to a transmitter, meaning that even established networks have had to build new masts.