No voice between Voipswitch and Topex,Please Help

Hello all,

I am having an annoying problem with the Voipswitch sending calls to Topex Multiaccess GSM gateways.

When I send calls to the Topex GSM gateway with the Voipswitch, I get no voice. However if I send from Voipswitch to a Hypermedia GSM gateway, I get two way full voice. Also when I send from a Nextone switch to the Topexs, there is 2 way voice!

I have troubleshooted endlessly with Voipswitch support and Topex support, and both are saying that the problem is not with their product. Please note I have tried the following:

1. I have tried with and without fast start and H245 tunnelling
2. I have enabled all UDP RTP ports on Voipswitch and Topex
3. I have disabled firewalls and NAT
4. I have tried every single signalling mode in the Voipswitch,with and without proxies, etc

To recap:

1. Im trying to get voice between Voipswitch and Topex Multiaccess Gateways, it gets no voice
2. Voipswitch sending to Hypermedia GSM gateways has voice, so it seems voipswitch is ok
3. Nextone switch sending to Topex also has voice, which indicates that the Topexs are fine too!

Please help me I have spent thousands of dollars on this set up and I've reached a dead end. Any help is appreciated.

Thank you