When you start a phone call on the Pipedrive app you will be prompted with the call summary view after the call ends. On the call summary view you can fill in the call notes and schedule followup activities.
Logging the call will only work if it's started from within the Pipedrive app and lasts for at least one second.
You can start a phone call on Pipedrive app by tapping on the phone number on the contact person view or on the call icon on a deal or activity view.
The call summary will be saved as call-type activity on the associated person's, and deal's, detail view.
This feature can be disabled by clicking More > Preferences, then switching off call logging.