asterisk users - Nov 2006 - can't hear MusicOnHold when zap answers

Hello,

Using 1.2.13 with bristuff:

exten => 8599,1,Answer()
exten => 8599,n,Wait(1)
exten => 8599,n,MusicOnHold(default)

Whan the call comes through a zap (telco) channel I can't hear the 
music, but through a sip/iax channels I hear it.

Any idea why?

Thanks,