In the Client, go to Call and then Recent:

Select a number/call from the list, to get to the view where you see all previous calls with this number. To add one or more labels, you click the Add a label menu (1) and to add notes you simply enter your notes in the Add notes field (2) – the labels and notes will be saved automatically.

In this view, you can also, if you want to, save the contact information by creating a new contact or save the phone number to an existing profile. Just click + Create new contact or Add to Existing Contact and enter the contact information for the person, and the client will remember this until the next time the person calls.