You won't get the second phone call.
The first call that you get was correct in that you'd receive a second call after you had collected about 10 Golden Gnomes or so; that second call would tell you wehere to find a Gnome Bomb which you would then use to blow open the passage to the deep sewers where the Chamber of Gnomes sits.
When they launched the Trials of Gnomus update one of the things they did is automatically open that passage for all players who didn't already have it open but it would seem that they forgot to update the phone call that you receive from the Gnomes to remove any reference to the second phone call which they removed because it was no longer necessary with the passage opened by default now.
