Are you using chan_sip or chan_pjsip? If chan_pjsip then you need to use the PJSIP_HEADER function [1] instead. For instance the following returned the call-id for me:

PJSIP_HEADER(read,Call-ID)

How are you attempting to use the channel function [2]? For instance the following returned the call-id for me as well:

CHANNEL(pjsip,call-id)

[1]
https://wiki.asterisk.org/wiki/display/AST/Asterisk+16+Function_PJSIP_HEADER
[2] https://wiki.asterisk.org/wiki/display/AST/Asterisk+16+Function_CHANNEL