To tie a phone number/call to a customer card in Contacts, you go to the Call tab followed by Recent.

Click the three dots to the right of the phone number in the list. Select whether you want to create a new contact, under Create new contact, or if you want to add the phone number to an existing contact, under Add to existing contact.

Read more about the possibilities in Contacts in the following article: What can I do in the Contacts tab?