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Extending cellphone service to the subways was “very, very complex engineering feat,” Bayne said.

Most of New York’s subway stations were built 100 years ago out of heavy steel girders that block radio signals. Even the steps are covered in steel for durability.

Crews had to route fiber optic cables along city streets and install dozens of antennas. One of the trickiest tasks was ensuring the handoff between underground and aboveground antennas as a caller walks out of a station, Bayne said.

The business side was equally tricky, said Mark Bienstock, a planner in the transportation authority’s systems office. Rather than let several cellphone companies all put antennas in the subway, MTA officials decided to contract with New York-based Transit Wireless to build the system and rent bandwidth to cell providers. The company is majority owned by Broadcast Australia of Chatswood, Australia.

Building the system will cost Transit Wireless and the cell carriers between $100 million and $200 million, officials said. The MTA will get a share of the profits, a minimum $3.3 million annually, once the network is in all the stations.

Transit Wireless said it plans to expand service to 30 other stations within a year, including the busy hubs at Times Square, Herald Square and Columbus Circle.