A service launched this week unblocks Caller ID information that the caller thinks is blocked, causing concern among those who help abused women.

TrapCall.com lets people rig some AT&T Inc. and T-Mobile USA phones so that if a call comes in with blocked Caller ID and is rejected, it rings again, this time displaying the number of the caller.

The basic service if free. The Web site charges for plans with more advanced services, like call recording and Caller ID with name.

The Web site said the service can be useful for people who get harassing phone calls.

But Becky Moreno, victim assistance coordinator at the Warsaw, Ind., police department, said TrapCall could be troublesome for victims of abuse. They, or their representatives, often block Caller ID when reaching out to abusers. Victim centers sometimes block their IDs when they call victims as well, so abusive partners won't know who's calling.

"This program doesn't distinguish between those who really need to stay anonymous and those do just do it for other reasons," Moreno said.

Calls with blocked Caller ID have never been perfectly anonymous. Calls to 1-800 numbers are unmasked, for instance, because the recipient pays for the call. This appears to be the loophole used by TrapCall. TelTech Systems, the company behind the service, did not return a message for comment Wednesday.

Fittingly, TelTech has a solution for those who need to stay anonymous: SpoofCard, a service that lets callers assign a fake Caller ID when calling.