Denver is one of two cities experiencing delays in answering 911 calls due to a glitch involving T-Mobile phone lines, a 9Wants to Know investigation found.

Denver's 911 call center says it's a software glitch that left at least one caller on hold for five minutes earlier this year. The mystery glitch is called "looping" -- it happens when a call has come in and been handled, and then the same phone number gets repeatedly reported as a hang up call, or looped back into the system.

9Wants to Know asked Athena Butler, Denver 911 executive director, about the software issue that frustrates callers, 911 operators and leaves others waiting on the line.

"It can be very serious and very frustrating because while we are calling someone back who hasn't called us, someone else is calling in that needs help," Butler said.

Since the beginning of the year, at least one individual waited five minutes before getting through to 911.

"We have had some looping calls come in and it took us about five minutes to call someone back," Butler said. "They were on the phone for three minutes, they hung up. We were on other calls, after about two minutes, it dropped to somebody to call them back. That's how busy we were during that time period."

The looping calls usually cluster during the most busy times for 911 -- most often on weekends or in the evening.

When Denver 911 is notified that someone called and hung up, the policy is to call back to make sure the caller is safe.

But a reported glitch shows that when some T-Mobile calls come through, even after the calls are handled by the operators, the numbers will be reported again and again to the system as hang ups. When those calls get looped back, 911 operators have to return them, wasting time and resources.

Last month, the call center recorded only 18 looped calls, up slightly over the previous month, but down dramatically since September -- when Denver 911 started working with T-Mobile, the wireless company responsible for the looped calls.

"The key is, T-Mobile has come to the table, they have been very good to work with to try to resolve this issue," Butler said. "But until we get to zero looping calls, it's not resolved."

Denver 911 first recognized the problem in the fall of 2015. It wasn't until September 2016 - about a year later - when call operators really started keeping track of the number of looped calls. There were 127 repeat phone calls during the last two weeks of September.

"It has been frustrating," Butler said. "The problem has been to the point where... we have called some people back five, six, seven times, and they have been frustrated by that."

Still, the volume of repeat calls is down dramatically since the issue first began, and T-Mobile says they've addressed the problem.

"We take our customers' security and safety very seriously. We previously identified an isolated issue and, in collaboration with Denver 911, addressed it. We will continue working closely with Denver 911 and monitor the situation," T-Mobile said in a statement.

A similar issue has also been happening in Dallas. There, one state representative was on hold for 26 minutes waiting for 911 to answer her call -- as the 911 phone lines were reportedly flooded by repeat calls from T-Mobile.

In Denver, Butler wants the issue resolved before the summer months, when call volume increases dramatically.

She also advises callers, even those placed on hold -- not to hang up. It takes longer for operators to call back than it would for them to answer a call on hold. Denver 911 also takes text messages - simply enter 911 in the "To" space, and send them a text.

If you've had trouble with 911 calls -- either you've waited on hold or had an operator call you back multiple times, let us know. Send your story to