ICAO Doc 8168 - Chapter 3

OPERATION OF AIRBORNE COLLISION AVOIDANCE SYSTEM (ACAS) EQUIPMENT

3.2 USE OF ACAS INDICATORS
c) in the event of an RA, pilots shall:

1) respond immediately by following the RA as indicated, unless doing so would jeopardize the safety of the aeroplane;
4) as soon as possible, as permitted by flight crew workload, notify the appropriate ATC

Note 1.— Stall warning, wind shear, and ground proximity warning system alerts have precedence over ACAS.
Note 2.— Visually acquired traffic may not be the same traffic causing an RA. Visual perception of an encounter may be misleading, particularly at night.

When a RA occurs, the Flight Crew must notify the appropriate ATC unit as soon as possible, as permitted by Flight Crew workload, in the following manner: "Company 123, TCAS RA standby"

If the pilot is unable to comply with an ATC clearance or instruction because there is a RA, the appropriate response is: "Company 123 - Unable - TCAS RA Standby"

The ATC response in both cases should be: "roger"

This relieves the controller of the responsibility to provide separation and he will not try to modify the aircraft flight path until the pilot reports "CLEAR OF CONFLICT"

When TCAS announces "CLEAR OF CONFLICT" the PF must return the aeroplane promptly but smoothly to the ATC clearance. Announce the TCAS "CLEAR OF CONFLICT" message when the conflict is over using the phraseology:
"Company 123, CLEAR OF CONFLICT, RETURNING TO (assigned clearance)", or
"Company 123, CLEAR OF CONFLICT, (assigned clearance) RESUMED".