Past Simple and Past Continuous

Use the past simple for a finished action or series of actions in the past

The doorbell rang so I put down my book and answered the door.

To talk about an action in progress when another shorter action happened, use the past continuous for the action progress. Use the past simple for the short action. This action can interrupt or stop the longer action.

I was reading my book when the doorbell rang.

Notice difference between these two sentences:

He was speaking when the phone rang.
He spoke when the phone rang.