When the call page loads, all fields are editable with the exception of the call Outcomes. This is to ensure the "Start dialling" button is selected, as it's this action that saves the call start time to the Activity notes.
If there is more than one telephone number available for the Company and Contact, you will be offered the option to select any of the numbers from a list when hovering over the "Start dialling" button at the top of the page.
Once you're ready to call the contact, click on one of the numbers to fully activate the page.
If you have configured TAPI integration, your call will automatically connect.
In the event of logging an in-bound call, the option to activate the call page without dialling is provided at the bottom of the list, by default.