[asterisk-ss7] chan_ss7 Dead Air for ~20% of the calls

[Antoine Megalla]

Hello,
 
I have an Asterisk SS7 installation used for a prepaid card application.
Everything is sort of working fine, but I have a nagging problem:
 
for almost 20% of the calls the callers who dial in on the SS7 lines are getting dead air, they cannot hear the IVR prompts, no audio can be heard on the line howerver if I enter the DTMF digits, the menus continue, but still no audio in any IVR menu, for the other 80% of the calls the system is working fine.
 
The setting is as follows:
Asterisk-1.2.24
asterisk-addons-1.2.7
Zaptel-1.2.20.1
libpri-1.2.5
chan_ss7-1.0.9
 
Red Hat Enterprise Linux Server release 5
kernel 2.6.18-8.el5xen
 
Digium TE410 card
4 E1s are used, first E1 carries the signalling channel on TS 16
 
I am not sure if this linked to chan_ss7 or zaptel, I have no way for testing this, and everything looks good on the console, no problems are reported.
 
I would appreciate it if anyone can point me in the right direction for solving such a nagging problem.
 
Thank you and best regards,
 
Antoine Megalla.
SAND (S.A.E)