I have been working with Cisco VoIP since 2001 and Asterisk and freepbx ,trixbox, elastix since 2007.
I'm from pakistan, it will be easier for me to debug the voice issues,
which isp are you using ?
are you connecting directly to the sip server or using some vpn to connect?
That being said I would like to have a little chat about your project to better understand the needs of the project
regards,