Ok so here’s what I’ve done
Copied one of my existing trunks - the registration setting is set to none
Override the trunk dial options to use what you mention
Added the below to extensions_custom.conf
[custom-privacy-header]
exten => s,1,Set(PJSIP_HEADER(add,Privacy)=id)
exten => s,n,Return
I then assigned this to my Anonymous Outbound route, with dial pattern *67 and override extension to hidden set?
Tested dialing my mobile from softphone prefixed with *67 and the call failed