the main thing u need to consider in voip bandwidth
is the codec in use
for example with g711 voice call consude about 64k while with g729 24 this is whiout over head
one more idea
if u are using CAC with CCM one call g711 needs 80k and g729 24k
while with an IOS gatekeeper
g711 needs 128k and g729 16k per call
all above without L2 overhead
good luck
if helpful Rate