Step 1: While on an active call, click on the magnifying glass that appears on the right side of the phone number located on the right-side panel.
Step 2: Search for the lead, Contact, Account or Person Account you want to relate the call to.
Step 3: You should decide if you want to just 'Save' the call and relate it to the object or 'Save & Merge Number' and also add the other party's number to an existing Salesforce record, where you'll get to select the phone field you want to update in your CRM.