NCNG is also a good one to remember; I personally have practiced it many times as a controller but not recently. Just saying 'turn right/left now' and 'stop turn now' while you count 4 sweeps of the radar (at 15 rpm) gives about 45 deg of turn.

My base uses the speechless code during most of our TRUCE sessions. We have our own target aircraft and combine speechless with NCNG and simulated emergency to short-pattern emergency SRA. We work our ATCOs pretty hard. The hardest thing is not to reply so I pull the mic plug out. After the code is explained the first question should really be 'is this a practice?' !
Clearly chevvron likes to get the occasional . . . . for homing. Maybe his new LARS trainees would like some out of the blue? :E

Also my experience. In fact I've found the two clicks as acknowledgment pretty standard practice, certainly on the continent and in the US. Could it be that - once again - the UK is at a variance with the rest of the world ;) ?

Anyway, easy to remember - if speechless here, just click the other way :)

two clicks
 

apart from being taught that (under the Oz syllabus) two clicks for affirmative, two clicks is intuitively more sensible - one click could be a mistake - two seems more of a definte action to me.

Now you come to mention it ... yes I was taught this, maybe in 1989, and have never used it[#] and have forgotten it. So yes, a reminder would be helpful.

I have, however, remembered some stuff about squawking 7600 and flying past the tower waggling wings and looking for a green light. (I've usually got a switched-off phone with me; I'm not sure I would bother to turn it on and call anyone - perhaps if I had a passenger with me to look up the phone number in Pooleys I might.)

And, probably most useful of all, I most certainly remember that the aeroplane doesn't know that the radio isn't working, and will keep flying perfectly happily.

[#] I once got "station calling xxx, carrier wave only" when calling for taxy, and that was enough for me to diagnose and fix the problem (headset plugged into wrong sockets).