There are only certain areas that will receive a signal. We tend to use and rely on sat phones or Spot Messengers for communication during our 5 day trek covering the main ridge of the Sierra Nevada.
I have made some notes intended to help walkers, trekkers and mountaineers heading into the mountains.
Mobile phone
(2) The summit of Mulhacen (3482m) can be relied upon to send and receive calls and text messages.
(3) The summit of Alcazaba has faint intermittent coverage. I have found if you wait long enough a text message will eventually be sent.
(4) Siete Lagunas. No signal generally around here but sometimes on the lip of the valley above Chorreas Negras a signal is obtainable.
(5) The hut doorway of the Poqueira hut is much better than inside the building, where I have never managed to get a signal.
(6) North of Alcazaba don't expect much until descending from the Picon de Jerez.
(7) On the descent from Siete Lagunas to Trevelez a signal appears at the ruined Campanuela refugio.
(8) The Poqueira Gorge, Genil, Trevelez, Lanjaron and Rio Chico valleys have poor or non existent coverage.
This is not a definitive list. If you have any contribution to add, or know of any changes, please let me know.