Johnson also wants to ensure the mobile networks shoulder the costs themselves and not push them onto consumers.
The Greater London Authority told City AM that mobile use on the tube "remains dependent on the market providing a credible proposal that satisfies both technical and commercial considerations".
Last year, Transport for London (TfL) scrapped a six-month trial that would have seen mobile signal available on the Waterloo & City line due to high costs.
A TfL spokeswoman told silicon.com that none of the proposals they had been presented with were "commercially credible".
"While it is technically possible to deploy mobile-phone and data-wireless solutions on the deep-level Underground tunnels and stations, the unique nature and environment of the Tube mean that project costs would be prohibitively high at this time," she said.