BOSTON — Nashira Muniz was sending a text message on her cellphone the other day when the phone rang. “I just texted you!” she said to the caller. Nothing unusual about that, except Ms. Muniz, 25, was deep underground, riding the subway.

She then checked on her child at home, made a banking transaction, received a few Facebook notices and arranged to meet another friend — all from the subterranean depths.

Last month, Boston completed installation of cellphone service — at least for T-Mobile subscribers — along all 11 miles of the Orange line, the first of its four subway lines to have end-to-end coverage. But the service has highlighted a problem, and it is not the expected one of fellow passengers yakking loudly.

The bigger problem, transit officials said, is cellphone theft, a growing concern throughout the country. Thefts occur even in subway systems that do not have cellular coverage because passengers display their phones as they read or listen to music and hope to catch the occasional signal that leaks through a street grate or when the train goes above ground.