There have been discussions going on regarding the installation of cells to enable mobile reception - one of the arguments that has been marshalled against this is the possibility of cell-phone bomb detonation!
1)the few stations that are either open or very close to ground level. Edgware Road Circle/District/Hammersmith and City line station is one of them. Otheres where reception is possible include Earls Court, South Kensington, Barbican and Whitechapel.
2) The sections of track open to the sky where the line was not rebuilt over when it was constructed. There is a long section going east from Kings Cross on the District/Circle line where this happens but there are small sections all along the north-eastern section of the Circle.
However, these isolated areas of reception are too unpredicatable to allow for the detonation of the two on the higher level lines and the "Kings Cross" bomb was in a deep tunnel where there are no signals.
