one way audio problem with Sysmaster Gat

We have a Sysmaster Gateway and VoiceMaster that uses H323.After our recent upgrade,the Ip phone which was working perfectly well before no longer works.
At moment the we can hear the Receiver but the Receiver can not hear us.The Support at Sysmaster have done all they know but the problem persists.
The ip phone also uses h323.
Kindly assist us if you have any idea of what needs to be done.

Osy