1.
Optimize the range of the mobile network.
See this Apple article: If you see No Service or Searching on your iPhone or iPad
https://support.apple.com/en-us/HT201415
2.
You need strong Wi-Fi, see the following Apple articles:
Connect to Wi-Fi on your iPhone, iPad, or iPod touch
https://support.apple.com/en-us/HT202639
If your iPhone, iPad, or iPod touch won’t connect to a Wi-Fi network
https://support.apple.com/en-us/HT204051
3.
See this Apple article: If you can't make or receive calls on your iPhone
https://support.apple.com/en-us/HT204027