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 other end of the connection.
Check the following:
- Check the signal strength that is indicated by the signal indicators at the left side of the phone's screen. These indicators move up as the strength of the signal increases and down as the signal decreases. Try moving to an area with better reception. The phone automatically scans for the best available network.
- Remove any covers or shells from the handset temporary to check if covers affect the phones's signal strength.
- If you noticed dropped calls at a specific or a few specific areas several times, it is advisable to contact the local operator and tell them what is the exact place where this happens.
- Check to see if a touch screen stays active while in the call, if this happen you may accidentally press "hold call" or "end call" button on the screen with the cheek.
- Try your SIM card in another phone to make sure that it is not an issue in your phone (this also exclude the conflict of 3rd party applications).
- Make sure your handset is using the latest version of software.
It is possible to fix many phone problems yourself in a few minutes, use the search field on this page to find the article: What are the simple troubleshooting steps to try when my Nokia Lumia phone is not working properly?