Hey Sushant, Spiti valley is very bag with for all networks except BSNL network ... both pre and post paid works well in Spiti valley. If you are travelling from shimla aise jio, vodafone and airtel works well for both internet and calls till Reckong Peo almost till the end of military settlement der. Once you cross that it's only BSNL which connects you to the outer world. Even BSNL won't save you between losar and gramphoo, so you are without any connectivity in kunzom top, Chandertal, Batal and Chatru .... See that BSNL accompanies you for the major part in Spiti valley. there are many bike rental and car rental companies in chandigarh and delhi who also give away Der BSNL sim fot your trip. On my trip to spiti Joginder Singh of Rana Cabs had provided me bsnl prepaid sim. Happy Travelling bro !!!
Spiti is one of the remotest region in the country and remains disconnected from rest of the world during winters. Due to such adverse geography and weather conditions, no other cellular network (except BSNL) will work in Spiti region. The BSNL will also work in and around Kaza (the headquarters of Spiti Valley). Both prepaid and postpaid network will work in case of BSNL but not mobile data. Networks such as Airtel, Jio and Vodafone will work till Rekong Peo (coming from Shimla) and Gulaba (coming from Manali). There are few points after crossing Rohtang Pass where you can find signals.