Voice is not clear with Asterisk server

Hi Friends,

I am working with Asterisk server. I am connecting two analog phones to one of ONT board. The board has an IP address and the two phones working with two extensions as 277 and 278.

Now i am trying to pass the all RTP packets through Asterisk server. When i call one extensions to another, the first few words(the first two hello’s) are getting clear and from there i am getting noise and the voice is getting delay.

Even i change the Jitter option. Even then i am not able to over come this problem.

Is there another way to overcome this problem.