Access by outside agencies* to the Department's radio was granted by the New York City Police Department well before 9/11, and vice-versa. While, as '3825 states above, generally speaking, outside agency users will identify themselves in the clear, there are instances where select outside agencies do not wish to use NYCPD channels and ID themselves in the clear. In those instances "RMP" numbers have been issued to those agencies so that when they key up, their traffic will sound like a typical NYCPD unit - and the same applies in reverse, in other words there are select NYCPD units that have dual radio IDs, one for use on their home system, and one for use when using outside agency channels.
* Not just DSP, Port Authority of NY & NJ, and AmTrak Police, but also:
FDNY*EMS
NYC Office of Emergency Management
NYC DoF Sheriff's Office
NYC DHS Police
NYC DoITT (although, now that the agency has been disolved, I can't confirm that the new agency has access anymore, but I suspect they do).
NYC Department of Investigation
NYC Department of Probation
NYC Department of Transportation
The assorted County District Attorney's Offices
Bergen County Sheriff's Office
Mount Vernon Police
Nassau County Police Department
MTA Bridges and Tunnels
NYS Department of Corrections and Community Supervision
NYS Environmental Conservation Police
NYS DHSES
NYS Park Police
Assorted US DHS agencies
Oddly enough, I have nothing in my notes showing that either the City's Health and Hospitals Corporation Police or any of the Sanitation Department's law enforcement agencies have / had access to NYCPD channels, despite both of them having / had UHF capabilities.