Dippy, just for, I went and searched my library and found the document.
By my standards, it's up to date, 1976 !
(yes, that's right, POST war).
According to my copy, the rule about no encoding OR "secret language" is applicable to ANY voice only channel.
That was when the GPO was in charge, maybe things have changed a little since ;-)
P.S.
If you have been listening to my calls, then you'll know that I've also been listening to yours.