This is a very much rough demonstration of CCIR493 selcall.
I used a pair of Jenal SC2 microphones, one per radio.
The radios are a Xeigu G90 and Icom IC-7200.
Both radios are connected to dummy loads, and are running 1W.
There's just a tiny amount of leakage to allow the radios to hear each other, but nothing radiates beyond the confines of the shack.
Jenal is no longer in operation, but their old website is still up for documents etc. Jenal
I have some info on my Wiki (GM4SLV Selcall stuff)[https://g4slv.info/dokuwiki/doku.php?id=public:radio:topics:ccir493_selcall]
The HFLink group have info at (HF Link Selcall)[http://hflink.com/selcall/]
If you have any Codan, Barrett or similar gear with HF Selcall I'd be happy to do some experiments.
Drop me a line at the email address (shown on my QRZ.com page) for G4SLV