Some London Underground passengers travelling between parts of the Central line are now able to receive 4G phone coverage.
EE and Vodafone customers between Holland Park and Queensway stations can get coverage in ticket halls, platforms and in the tunnels.
The Northern line between Kentish Town and Archway will follow next week.
Transport for London (TfL) says it will be "fully connecting" the whole Tube network by 2024.
Phone networks Three UK and Virgin Media O2 are also taking part in the 4G and 5G-ready rollout, with customers gaining access in the new year.
Central and Northern line stations Oxford Circus, Tottenham Court Road and Camden Town will also follow in the new year.
Previously, the only underground tunnels to get phone coverage were on the eastern half of the Jubilee line between Westminster and Canning Town.
Once fully delivered, more than 2,000km of cabling and thousands of radios are expected to be installed within tunnels and stations.
About 500 people are working overnight across the Tube network to install the equipment, TfL says.
TfL awarded a 20-year deal to 4G and 5G infrastructure company BAI Communications who are overseeing the project.