Updated : July 2017
If staying connected with the outer world is important for you while you are travelling to Spiti valley, following table will help you prepare better. I always carry a BSNL/MTNL Postpaid with me along with Airtel Postpaid.
In Spiti valley, only BSNL (both prepaid and postpaid) works in roaming.
Table below is based on my usage and experience and it keep changing frequently in terms of new areas added or no signals because of no power or any landslide breaking the cables.
So plan your connectivity needs in advance, complete or delay any urgent communication before or after the trip.Data speed is 2G right now, but work is progress of the 3G cabling.
Note : I have no association with Airtel, BSNL or Reliance Jio, this is my experience based on what I am using right now.