If Network is weak as you've indicated in your area then call connection's can have issues unfortunately.
Restarting your phone of putting it in flight mode will help refresh Network connection which is why things seem better for a while.
I assume this issue isn't happening everywhere or when you have good network connection ?
Unfortunately until your chosen network improves network signal where your experiencing issues there isn't much you can do.
Apart from perhaps changing the network you use or pop in another network sim card in sim slot 2 so you can switch to the other network if required.
If Calling Over Wi-Fi switching is causing issues then either turn that off in your Network app and phone or turn off Wi-Fi in your phone's settings.
I wish you all the best. 
