In a statement on its website, Vodafone underlined what it saw as a weakness in EE's 4G - indoor coverage.

BBC: UK's first 4G mobile service launched in 11 cities by EE

When EE's network launched, rivals questioned whether the spectrum band that it was using - 1800MHz - would provide good indoor coverage.

BBC: 4G tests reveal patchy urban coverage

The mobile firm has made no secret of its desire to get its hands on lower-frequency spectrum, which can cover much larger areas and offers better indoor coverage than the high-frequency spectrum it currently owns.

BBC: Mobile firm on rural net crusade

"Indoor coverage matters, " the company wrote.

BBC: UK's first 4G mobile service launched in 11 cities by EE

TOKYO, JAPAN, November 16, 2012 --- NTT DOCOMO, INC. announced today that it has developed the world's first dual-mode small-cell base station, or femtocell, supporting 3G (W-CDMA) and LTE simultaneously for improved service coverage in indoor locations such as offices, shops and homes.

ENGADGET: NTT DoCoMo readies first dual-mode HSDPA, LTE femtocell for December

In China, lightRadio Metro Radio will be deployed in busy indoor and outdoor locations, such as shopping malls and stadiums where coverage can suffer either from the large number of consumers trying to access a network, or because the density of buildings impedes the radio signal.

ENGADGET: Alcatel-Lucent's latest lightRadio development aims to spread TD-LTE across China