5.2 Radiotelephony procedures
When Selective Calling (SELCAL) equipment is used certain of the following procedures are superseded by those contained in 5.2.4.
18.104.22.168 PANS. When a controller or pilot communicates via voice, the response should be via voice. Except as provided by 22.214.171.124, when a controller or pilot communicates via CPDLC, the response should be via CPDLC.
Source: ICAO Annex 10.2