I think the sound quality is bad. Why does that happen?#383

Dropped calls or low sound quality during calls can result from low signal strength or momentary gaps in network coverage, either at your end or the receiver end. In most cases, your operator or service provider is the best source of assistance with issues regarding signal strength.
Signal strength is indicated by the signal indicators at the left side of the phone's display. These indicators move up as the strength of the signal increases and down as the signal decreases. To increase your phone's signal strength, try moving your phone slightly, or move towards a window if you're calling from inside a building.

If you continuously experience low signal strength in a certain area, it may be that network coverage is insufficient. Your operator or service provider can provide you with more information on network coverage.