When you speak to a contact or lead, you can log a call activity to that record. Logging calls will let you know, at a glance, whether you have or have not recently contacted this person via phone.

To add a call for a record, scroll down to the Activity Timeline of the record and click New Activity -> Call.

Enter the subject and date/time of the call. Enter any notes taken during the call.

Once you are done, click Save. The call will now be saved to the timeline of the record.