B737-800
Flight Crew Multiengine; Flight Crew Air Transport Pilot (ATP); Flight Crew Instrument
Flight Crew Last 90 Days 349; Flight Crew Type 20000
Communication Breakdown; Confusion; Situational Awareness
Deviation / Discrepancy - Procedural Other / Unknown
Air Traffic Control Separated Traffic; Air Traffic Control Issued New Clearance
Company Policy; Human Factors; Procedure
I was always under the impression that the PRM (Precision Radar Monitor) Monitor is there to manage any converging traffic issues and issue break out instructions as described in the ALL USERS page. That is all I ever expected to hear on the PRM frequency. In ZZZ; though; they seem to do way more than that. I've had PRM Monitors ask me for a radio check before; as if checking just to see if we're complying with the procedures. Today I had one really mess me up because inbound to the runway; and already on Tower Frequency (female voice); I heard another person tell us to slow to 170 knots. (Male voice) I answered on Tower and there was no other comment. Then the male voice said 'Slow to 160 knots.' The First Officer then said he thought that was on the PRM Monitor Frequency. I then replied on the PRM Frequency and got no response. A little bit later the male voice; which was the PRM Monitor because I could tell by raising the volume; told us to slow to approach speed. Never once did that voice identify itself as the PRM Monitor. At the least this caused confusion. Usually if there are separation issues on final; the Tower will tell us to slow. Having the PRM Monitor doing that just doesn't seem correct.
B737-800 Captain reported confusion due to being assigned speeds from the Precision Radar Monitor Controller while also on Tower frequency.