Malicious Caller Identification (MCI), also called Malicious Call Trace (MCT) or Caller Activated Malicious Call Trace (CAMCT) is a facility which enables a phone call recipient to mark a phone call as malicious (i.e. harassing, threatening, obscene, etc.) The phone system will then automatically trace the call. The call trace is made so it will work even if the call was made from a silent number

Silent number

In telephony, a silent number, unlisted number , ex-directory number or private number is a telephone number that is intentionally not listed in telephone books....

, payphone or a number with caller identification disabled; but the trace is only available to law enforcement. Generally, law enforcement will only act on the trace once a formal police report is filed in regards to the call.

How the user activates this feature depends on their phone system—generally it is either a DTMF sequence or a special button on their phone. For analogue services, many exchanges will interpret DTMF sequences to activate POTS

Plain old telephone service

Plain old telephone service is the voice-grade telephone service that remains the basic form of residential and small business service connection to the telephone network in many parts of the world....

. Digital services, such as ISDN or GSM, enable the CPE

Customer-premises equipment

Customer-premises equipment or customer-provided equipment is any terminal and associated equipment located at a subscriber's premises and connected with a carrier's telecommunication channel at the demarcation point...

 to activate MCI via out-of-band signaling. Often with PABXs with analogue phone connections, the PABX will interpret the DTMF and then activate the out-of-band signaling.

MCI is the ETSI standard, whereas MCT is the American one. They both do more or less the same thing though.

In Canada, the service is usually marketed as Call Trace, and fees generally only apply when the service is used.