Subchapter 9.4 – TCAS Resolution Advisory and Safety Alerts

Responding to a TCAS Resolution Advisory:

When you hear "Climb, climb" or "Descend, descend," you must:

Initiate the commanded maneuver immediately. If TCAS says climb, pull back and climb at the recommended vertical speed. If it says descend, descend at the recommended rate.
Inform ATC of the RA: "Zurich Approach, HB-CDA, TCAS Resolution Advisory, climbing."
Follow the RA to completion. Do not stop or reverse the maneuver until TCAS says "Clear of conflict" (or the aural message stops).
Once clear, return to your previous clearance or request new instructions: "Zurich Approach, HB-CDA, clear of conflict, returning to Flight Level 120."

ATC understands TCAS RAs and expects this response. They will not be surprised by your altitude deviation because TCAS safety takes absolute priority over ATC clearances.