SAN JOSE, Calif. >> Congested phone lines created by the merger of United Airlines and Continental Airlines caused Jill Lucas Mertely to break down in tears after spending 18 hours on hold over four days as she futilely tried to book flights to Hawaii using mileage points.

“It’s been absolutely horrific,” said the unemployed Felton, Calif., bookkeeper, echoing the experiences of other customers across the country. “It’s nothing I’ve ever experienced.”

United, which merged with Continental Airlines in 2010, shifted its entire reservations system to Continental’s on March 3, creating headaches for travelers and dinging the brand of the world’s largest airline. Though the airline said it was prepared for the switch, new glitches keep appearing nearly two weeks later, said Joe Brancatelli, who operates JoeSentMe.com, a website for business travelers.

“Up until the day before (the systems switch), United said, ’No problem. We have everything covered.’ Total arrogance,” said Brancatelli, who said his inbox is overflowing with complaints from road warriors.

In a message posted on its website on Monday, United noted that the conversion is the largest in aviation history and that “the vast majority of our systems are functioning as planned.” The company said it is working to reduce wait times, adding, “We apologize if you have had difficulty with your travel planning, and we are grateful for your patience.”

United spokesman Rahsaan Johnson said the company hired an additional 600 agents to handle the flood of calls, much of which he attributed to glitches with the online system and customers confused about its new website, which uses Continental’s old site as a template. Most of the problems have been tied to incorrect passenger information on the airlines websites, such as upgrades not being listed or itineraries getting jumbled, he said.

“Operations have been smooth in terms of checking in, boarding, flights departing on time,” Johnson said. “The issues we are having to work on are customers’ abilities to get the information they want online so they don’t have to call.”

Marc Casto, president of Casto Travel in San Jose, said the airline’s reservation-system conversion has been smoother than similar moves at other airlines. “They moved over hundreds of thousands of reservations overnight while planes were still flying, and everybody got to their destinations safely,” he said.

His company, which does as much as $30 million in business a year with United, worked with the airline for a year to prepare for the transition and was able to ease the turbulence for many of its customers, though his agents have also spent one to two hours on hold when they needed to speak with United. Some fliers are upset after losing some benefits when United combined its Premier status with Continental’s Silver level to create Premier Silver status, Casto said.

Still, he said, “I was expecting we would have multiple (flight) cancellations from San Francisco on an hourly basis. It was almost a non-event compared with what we thought it would be.”

The experience is just another reminder of the indignities air travelers endure these days in the United States, said Alan Bender, professor of aeronautics at Embry-Riddle Aeronautical University in Daytona Beach, Fla.

“I fly 150 days a year and I have minor versions of this happening all the time,” he said. “As the airlines get bigger and bigger, it is getting worse and worse.”

Brancatelli said United appears to have offended its most important customers — business travelers. The system didn’t recognize the status level of a number of elite travelers and other high-mileage travelers who upgraded seats online using the old United system, and then discovered the changes weren’t recorded on the new one.

“When you check-in online, it says call United. But you can’t call United. The waits are three, four hours, or they hang up on you,” he said.

“It’s not been fun,” said Cupertino, Calif.-based software analyst R. Ray Wang, who logs about 300,000 miles a year and has United’s ultra-elite Global Services status. United airport agents have not received enough training on the Continental system, he said, while Continental ticket kiosks could not process United tickets.

“Customer service is nonexistent,” said Wang, who is now booking flights on other airlines, including the one he took to London this week. “They are like a 9-to-5 operation in a 24-7 world.”

While many business travelers whose companies have corporate deals with United and depend on its extensive routes in the United States and abroad won’t abandoned the airline, others, such as entrepreneurs and venture capitalists, have more freedom to switch, Brancatelli said.

“If you are a United Global Services flier and you are walking away, that is really damaging because you spend a lot of money,” he said. “If Global Services members are walking away, they will feel it on the bottom line.”

Lucas Mertely admitted she had not been paying attention to the news about United changing its reservations system when she decided to book a trip to Hawaii using mileage points for herself and her husband. As a result, she spent hour after hour getting transferred, waiting on hold and having the system repeatedly hang up on her. She spoke with more than 16 agents, but none could figure out how to make the reservations.

“They try to transfer you and you get disconnected,” she said. “Or they try to tell you something that isn’t true. Or they literally say, ’I’m sorry. We can’t help you.’ ”