Roland Perry wrote:
"Transport for London confirmed to the Guardian that 4G mobile
phone technology would go live in tunnels on most of the Jubilee
line from March 2020 and on other lines in the coming years."

https://www.theguardian.com/technolo...alling-at-all-
stations-tube-passengers-to-get-4g-reception-from-next-year

A spin-off from the 4g replacement for Airwave, it says. Which could
mean only EE customers will benefit. [Heigh ho, another reason for
getting a dual-SIM phone].

I'm not clear how you work that out? The article says:

"While TfL is picking up the bill for initial trial on the Jubilee line
extension between Canning Town and Westminster, it will soon award a
contract to a private operator which will install 4G equipment within all
of London’s tube tunnels by the mid-2020s. Mobile phone networks will then
pay the private operator for access to the network, with the transport
authority receiving a cut of profits."

That implies that the kit will be installed by a third party, who then
charges mobile phone operators to use it, and shares the profits with TfL.
There's no suggestion that only one operator will have access. One way or
another, I'm sure all the major, and probably all, operators will provide
connectivity in the tunnels.