AMC2 SERA.8035 Communications
CAA ORS9 Decision No. 1
ACKNOWLEDGEMENT OF MESSAGES
(a) When a CPDLC emergency message is received, the controller shall acknowledge receipt of the message by the most efficient means available.
(b) Except as provided by (a), when a controller or pilot communicates via CPDLC, the response should be via CPDLC. When a controller or pilot communicates via voice, the response should be via voice.