The family has 2 kids.
BB represents the case for older boy and younger boy, and BG, GB, GG are similar.
b represents the case that hearing the voice of a boy during a call.
Assume that the voice is of a kid of the family.
The problem is what is P(BB/b).

Solution:
Assume that each kid has the same chance to shout and be heard during the call.
P(b/BG) = 0.5
P(b/BB) = 1
P(b/GB) = 0.5
P(b/GG) = 0
and P(BG) = P(BB) = P(GB) = P(GG) = 0.25

P(BB/b) = P(BBb)/P(b)
= P(b/BB)*P(BB)/(P(b/BG)*P(BG)+P(b/BB)*P(BB)
+P(b/GB)*P(GB)+P(b/GG)*P(GG))
= 1*0.25/(0.5*0.25+1*0.25+0.5*0.25+0*0.25)
= 1/2