How to get them to call?
the sound of running water.
when turn off the vacuum cleaner after cleaning their cage.
when Lavender chicks fledge (bit hard to arrange that one
but the birdroom goes mad).
let another Lavender loose to fly around the birdroom
(works a treat).
Call as gender determinant?
Determining cocks is fairly straightforward - a few minor irritations?
Firstly, cocks can be suppressed when in the company of a more dominant cock.
Remove the dominant cock and another may well burst into 'song'. I have direct experience of this.
So, identify a cock bird, remove him and wait a week or so to see if another declares himself.
Secondly, there are variations.
Some cocks make it harder by only doing the first of the
two part call. But they will come out of their shell if they are the only cock in the cage).
Thirdsly, cocks in a settled pair, housed alone, call very infrequently (if at all).
Finally, some cocks seem to be shy/ quiet (but they will come out of their shell if they are the only
cock in the cage).
In essence, the solution to determining gender from calls does involve moving birds around,
Cage dividers make it easy. It's not difficult.