If you have a refundable ticket, you must call UA before flight time or lose the value of the ticket. Way back, some of us would visit the PCs to drink and hang out (before gate passes for PC members) and the MO was to buy the cheapest refundable ticket and go to a service area after clearing security and have them 'Uncheck you in'. They would then issue the refund as well, if that was what you wanted.