U.S. airlines are scrambling to add staff and upgrade technology as they face anger over prolonged call center wait times while tackling a surge in air travel following COVID-19 vaccinations. "#Delta 9 hours wait on hold is this a way to run an airline," read one Twitter post from a Delta Air Lines (DAL.N) customer. After a year of being cooped up, travelers are balancing the prospect of sunshine in Florida or fresh mountain air in Montana and Wyoming - among the fastest growing U.S. travel markets - with frustrations during the booking process. By July, U.S. domestic air fares and capacity could approach pre-pandemic levels, according to experts, but overall staffing at the three legacy carriers shrunk by roughly 20% last year. While travelers can easily book new vacations online for flight changes or travel credits -- transactions that have soared during the pandemic -- they often need to go through call centers, which are also managing a flurry of questions about COVID-19 travel restrictions and requirements. To support the increase in call volumes, Delta is adding staffing and overtime, hiring temporary summer contract workers and fast-tracking technology upgrades to self-service options, a spokesman said. "Our wait times are not currently where we"d like to them to be," he said. American Airlines (AAL.O) said it is hiring hundreds of reservations agents to help with the rise in calls, while United Airlines (UAL.O) said it is working to shorten hold times through increased staffing and technology upgrades, without providing details. U.S. airlines received billions of dollars in government aid to pay salaries and protect jobs during the pandemic but also encouraged workers to take voluntary leave packages to slim staffing because they didn"t know how long the crisis, or the government aid, would last. Willie Walsh, the head of the International Air Transport Association, said on Wednesday that the decisions by carriers across the globe to retire aircraft and make staff redundant could hamper the aviation industry"s recovery. read more An explosion in U.S. domestic leisure travel demand as more Americans become vaccinated has taken many in the industry by surprise, leaving services from airports to rental car companies and hotels short-staffed. The strength of the rebound has possible implications beyond the airline industry since economists say air traffic is closely tied to overall economic output and is frequently seen as a guide to consumer confidence. However, analyst John Grant of flight data specialist OAG warned on Wednesday that U.S. domestic airline traffic may be "overheating," suggesting that legacy carriers are likely to reshape their networks toward more international markets once they reopen. Still, Grant said plans by U.S. ultra low-cost carriers and start-ups to build domestic capacity "has to be good news for all airports of all sizes across the United States in the next few years."