Phone ringing when no one is calling

[Godman_]

Why does my phone keep ringing when no one calling me? 

Please how can I stop this? 

[jrmL]

Did you try to restart the phone? Still persist after?

[Helenna]

Hi, after restarting the device if the issue persist again then please try resetting the settings on the device.

From settings>>General management>>Reset>>Reset settings>>Reset settings>>Tap on reset.

Performing reset settings will not delete any data, it just set the settings to default.