it's normal. it happens because u are in territory where your line keeps switching between 4g and 3g.
samsung phones don't have toggle for it, it's always on so it's just depends on whether the carrier supports volte for your device or not.
u don't have to restart to force it on, just simply turn on airplane mode for 10 seconds then turn it off. even if u dont have volte, u still can receive and make calls, so u shouldn't be worried much about it.
