The first Tubes stations in London's West End have received high-speed mobile coverage.
4G and 5G is available at Oxford Circus and Tottenham Court Road ticket halls and on the Central line between the stations.
Further sections of the Northern and Central line will be getting coverage in the coming months.
By December, 33 stations with underground platforms will have mobile coverage, increasing to 40 in 2024.
All four mobile network operators - Three UK, EE, Vodafone, and Virgin Media O2 (VMO2) - are taking part in the rollout, which has already delivered 4G on sections of the Jubilee, Northern and Central lines.
It is expected that by the end of 2023, the first Elizabeth line stations will have mobile coverage, with the whole line getting coverage by the end of spring 2024.
Mobile coverage will also host the new Emergency Services Network (ESN) which, when fully operational, will give first responders immediate access to life-saving data, images and information in live situations and emergencies on the frontline.
Boldyn Networks, the organisation rolling out mobile coverage on the London Underground, will also begin to upgrade the existing coverage on the Jubilee line to allow 5G to be available at stations, such as Canary Wharf and North Greenwich, for the first time.
About 500 people are working overnight across the Tube network to install mobile equipment. Once the rollout is completed, more than 2,000km (1,240 miles) of cabling as well as thousands of radios will be installed within tunnels and stations.