DENVER -- Since telephones started ringing, people have wanted to know who is on the other line before they pick up.

Even with answering machines and Caller I.D., some callers have found ways to remain anonymous.

In the past, there was no way of knowing who was on the other end of a “restricted” or “private” call.

But a new, free service called TrapCall unmasks that blocked number.

Matt Holmes, a student at The University of Denver had no way of knowing who was prank calling him from a restricted number, but the caller, claiming to be from Google MoneyTree, kept calling – up to 15 times a day.

“I said ‘Do not call me anymore. Remove me from your calling list,’” said Holmes. "If I wouldn't answer, they would say things like, 'Thank you for your donation.' Stressful things that just made me kind of paranoid."

He tried calling police, without a number "I couldn't do a thing. I called T-Mobile and they couldn't do anything either. I was completely defenseless."

But then, he discovered a new, free service online that promised to unmask blocked calls.

He signed up for TrapCall.

“When an incoming call shows up as ‘Unknown’, you push ignore,” Holmes explained, showing his phone. “The caller on the other line doesn’t know their call is being unblocked. They still hear the phone ringing, but in a few seconds the number will come back on your screen unmasked and you can choose to answer it or ignore it.”

Once users set up an account, the TrapCall service forwards calls to a toll-free number.

“In essence, because it’s a toll-free number, you have the right to know who’s calling you,” said Meir Cohen, president and co-founder of TelTech Systems Inc.

So, he said, TrapCall can legally unmask the once anonymous number and name.

Domestic Violence groups are concerned TrapCall could put victims at risk, especially those who must contact their abusers in child custody situations.

“The benefits of this technology do not outweigh the consequences,” said Denise Washington, executive director of the Colorado Coalition Against Domestic Violence. “If an ex-partner can use this new technology to find out my phone number, they potentially can find out where I’m staying.”

But Cohen said he made the service with victims in mind.

Matt Holmes said instead of feeling “defenseless” with every anonymous call, he has some power.

“The pranksters called me saying ‘Google MoneyTree,’ and I said, ‘Hey, I have your number. It’s this. Don’t call me anymore or I’ll report it to the police,’ And they haven’t called since.” he said.

TrapCall offers some paid packages with more bells and whistles, such as voicemail transcription.

A computer automatically transcribes voice messages and sends them to users in a text and an e-mail.

When 7 News tested the service, it worked, but some messages came more than an hour after they were left.

Cohen said they are still working out the bugs for some TrapCall services, but that with the free package, users will always know who is calling.

The creators of TrapCall have come up with another paid service for people who still want to remain anonymous.

SpoofCard allows callers to have a fake I.D.

TrapCall works with most cell phone carriers and in the coming weeks will be expanding to land lines.

Other free services, such as GoogleVoice, are on the way as well, as this technology continues to evolve.