Phone- Communication infrastructure in Nepal purely depends on places. The higher the elevation gets, the weaker the network becomes. Even though there are telecom companies that claim to provide a network in high elevations, the connection is not so reliable. Moreover, there are still trails where there are no facilities of any kind of modern communication method as well. So preparing yourself properly is something very much important before embarking on the journey.
Internet- Internet facilities are also completely reliable in remote areas. Famous trails in the Khumbu and Annapurna region do have WiFi facilities in many places, however you might have to pay a few extra bucks to utilize the service, and the connection might also not be stable due to chronic electricity shortage, weather, and elevation. For better internet service use both local SIM and WiFi. Whereas in cities, you will get WiFi for free in almost all hotels, cafes, and restaurants.
Post- Postal service is not as convenient as other means of communication, especially during traveling. It can take several days to reach the destination. If you are posting mail to the international address, make sure to leave it at the post office rather than in a post box. In Kathmandu, some of the bookshops do post your mail on your behalf, charging a small service fee. Likewise, you can also ask your travel company and hotel to post your mail.