WASHINGTON -- A suicide hotline operated by the Department of Veterans Affairs allowed crisis calls to go into voicemail, and callers did not always receive immediate assistance, according to a report by the agency's internal watchdog.

The report by the VA's office of inspector general says calls to the suicide hotline have increased dramatically in recent years, as veterans increasingly seek services following prolonged wars in Iraq and Afghanistan and the aging of Vietnam-era veterans.

The crisis hotline -- the subject of an Oscar-winning documentary -- received more than 450,000 calls in 2014, a 40 percent increase over the previous year.

Get Breaking News Delivered to Your Inbox

About in 1 in 6 calls are redirected to backup centers when the crisis line is overloaded, the report said. Calls went to voicemail at some backup centers, including least one where staffers apparently were unaware there was a voicemail system, the report said.

Hotline callers "made numerous complaints of long wait times for responders, being put 'on hold' or calls 'not being put through' to a responder," the report said.

Sen. John McCain, R-Ariz., said he was deeply saddened and disappointed by the IG report.

"The VA's failure to help our most vulnerable veterans is not only unacceptable, but it is shameful," McCain said. "The VA's inability to run a call center and deal with increasing demand has put our nation's veterans at greater risk."

A VA spokeswoman said the agency agrees with the report's recommendations and is working to modernize the crisis line to better serve veterans.

Deputy VA Secretary Sloan Gibson said in a statement that the agency is hiring more staff for the crisis hotline, which is located in Canandaigua, New York. At the same time, the VA has implemented staggered shifts to make sure there is more staff during peak call times from 10 a.m. to 10 p.m.

"We are improving our ability to be more available when our veterans need help the most," said Gibson, who visited the hotline center twice last year.

Upgrades are planned in the next year to provide state-of-the art phone systems and greater work space for staff who answer crisis calls, Gibson said.

About one-fifth of all suicides in the United States are committed by veterans, and the VA has highlighted suicide prevention as a crucial area of concern. A law signed by President Barack Obama year requires the Pentagon and VA to submit to independent reviews of their suicide prevention programs and make information on prevention more easily available.

The law also offers financial incentives to psychiatrists and other mental health professionals who agree to work for the VA and assists military members as they transition from active duty to veteran status.

An HBO documentary highlighting the life-and-death drama of the VA suicide hotline won an Oscar last year. The film, "Crisis Hotline: Veterans Press 1" won for best documentary, short subject.

Last June, CBS News did a story on Special Forces veteran Johnny Primo.

Not long ago, Primo logged on to his Instagram account and said he was horrified to see a suicide note from a veteran who lived just a few miles away.

The note read: "Very few people know the truth ... I want this in everyone's memory." Desperate to help, Primo tracked down his address.

"I was 45 minutes too late from him taking his own life," Primo told CBS News at the time. "Immediately it was a gut wrenching feeling, knowing that there was a chance that if he had my phone number he wouldn't have killed himself."

Primo called friend and fellow veteran Casey Gray and that night they launched an Instagram suicide prevention page by posting a message pleading with veterans who are having suicidal thoughts to call them, day or night.

They call it "22 Too Many" -- highlighting an estimate of the number of veterans who commit suicide every day. Veterans responded immediately.

"Within the first three hours we saved one person," said Primo. "Within the first 24 hours we saved five people, people who were on the verge of suicide."

Currently, "22 Too Many" has more than 17,000 followers on Instagram. Most of their posts are of people who are willing to take calls from veterans contemplating suicide.