On the 18th of July I heard the first calling sounds! Since then I can hear them quite regularly, amazing! I've been trying to record it but it always lasts rather short and/or happens in the middle of the night. It sounds exactly as you would expect; like the adults but much higher pitched, very cute! I am extra surprised because they hardly seem to grow anymore at all, still very very tiny! A time line: eggs laid on 26 April, first toad on land on 26 May, First time witnessed an amplexus on 2 July, First time hearing calling sounds on 18 July.