NOTE
A vehicle permit and an Inner Line Permit (for international travellers) are required to enter Spiti Valley. The permits are checked at Rohtang Pass. Also, make sure to take precautions to avoid altitude sickness and keep medication handy.
The network signals will be gone (for good!) as soon as you enter the valley after Rohtang Pass. It’s highly likely that you will stay entirely disconnected through most part of your trip, but you can always ask a local to let you make a call or two. You will always be entertained with a polite ‘yes’.