But that doesn't rule out the idea that only the specific sequence 9-9-9-call needs to work, whereas sitting on the keypad and generating 4-6-3-4-6-9-3-4-5-9-4-2-3-5-9-2-0-call could usefully say "stop being silly".

Yes, but laws (and the interpretations of laws) rarely take silly into account. Either that or they're just being lazy; I think, however, that they're mostly just covering their arses to avoid a lawsuit from someone trying to dial 999 whilst wearing boxing gloves.

or a timeout? leave the first 9 on the screen for X seconds and then give up and go back to a clear screen

A quick test suggests that mine only reacts to any substring being 9-9-9 (so 9-7-9-9 won't do).