According to the support pages of notable Android phone brands, dropped calls most often occur when you place your phone call in a dead zone or an area with limited to zero cell coverage. In order to have excellent call reception, you need to be near a cell tower. However, some obstacles may interfere with connectivity, such as physical obstacles such as tall buildings or trees. You may also be in a busy area where many people are also making phone calls. It can also cause connection issues and lead to calls ending abruptly.
If you think your location has something to do with your cell reception issues, consider moving to a less crowded location with fewer physical barriers. If desired, check a third-party website like CellReception.com to see if your provider has a dedicated cell tower nearby.
In keeping with coverage issues, your phone carrier may also experience some sort of outage. In some cases where the phone call has been going on for a while – intentionally or unintentionally – some networks may automatically terminate the connection after a few hours to prevent accidental calls and excessive call charges. Contact your mobile phone service provider and ask about any potential outage issues or check their coverage areas and other associated call termination protocols.