There are only a couple of places in the rules where a hail is required (or even mentioned). Those are RRS 19.1 (hailing for room to tack) and RRS 61.1(a) (hailing "protest"). Hails on the course of "starboard", "hold your course", "room", "no room", "proper course", "head up", etc. are not binding and don't enter into rule interpretation. Only actions count.

But (and this is an important but), such hails are still valuable. When two boats meet, it's best if both skippers know what's going to happen. That helps everybody to sail cleanly. After all, the right-of-way rules exist to prevent collisions. Hailing serves the same purpose. Oh, and if something does go astray and you wind up in a protest, having hailed may help show you tried to avoid contact.

Last weekend, I (sailing on port tack) called "hold your course" to a starboard-tack boat, just to be sure he saw me and knew I was going to keep clear.

Regards,
Eric