The two mobile network operators join the BAI Communications (BAI) network, alongside EE and Three.
As part of the initiative, customers of the four networks will have access to 4G and 5G mobile internet across underground platforms and tunnels.
Five London Underground stations will get 4G coverage in the next six months, Transport for London has said.
Mayor of London Sadiq Khan said improving the capital’s connectivity is central to London’s economic recovery from the Covid pandemic.
London’s mayor added: “I committed to Londoners that I would deliver 4G throughout the Tube network as part of my determination to build a better London for everyone - and I have no doubt that this will transform journeys for millions of passengers.”
Some sections of the Tube network will have mobile coverage by summer 2023, including parts of the Central line through the city centre at West End.
BAI was awarded a 20-year deal by Transport for London (TfL) to roll out phone and internet coverage so Tube users can scroll social media, schedule plans and even make calls at any time.
The Tube tunnels will also be laid with fibre cables to create a “full-fibre spine for London” as promised in Mr Khan’s 2021 mayoral manifesto.
It follows last year’s trial launch of 4G on the eastern end of the Jubilee line, between Westminster and Canning Town which will be incorporated into the new deal.
Free Virgin wifi already exists in Tube and London Overground stations.
Across the wider Connected London programme, BAI anticipates investing more than £1 billion to build “a backbone” of mobile and digital connectivity for the city.
Small mobile transmitter cells are also being connected to buildings, traffic lights and lamp posts to make 5G available in boroughs.