Now a day, there is cell phone reception entire trek even at Mt. Everest Base Camp (5,360m) and the top of the Kalapatthar (5,555m). You can buy Nepali SIM cards at Tribhuvan International Airport (TIA), even grocery shops,s, and other shops nearby your hotel. For this, you need a copy of your Passport and PP size picture with a page of form fill up, then you can easily obtain SIM Card, the cost will be NPR 100 or more depending on how much data you would like to buy. There are two different popular Cell Phone companies in Nepal, for the Everest Trek NCELL will be better due to good coverage in most places. If you need any help, we at Nepal Mother House are ready to assist you anytime.