Hi Sathya:
Just to be sure that we are talking about the same branch and not some other fork, are you using:
http://scm.sipfoundry.org/rep/sipX/main
The vanilla, open source version of sipX does not have an adaptive jitter buffer or a very robust packet loss compensation algorithm.
It you have network situations where you have any sort of packet loss or jitter, you probably will want to consider using SIPez's proprietary media engine plugin (SME) for sipX. With SIPez's SME, packet loss is not noticible to most people with up to 5% packet loss which is a pretty huge amount of loss for most network situations. SIPez's SME also minimizes latency which can build up sometimes with just the pure open source sipX solution. SIPez's SME is a pay for/commercial plugin.
Cheers,
Dan