Hi,
I have been working with Cisco VoIP since 2001 and Asterisk and freepbx ,trixbox, elastix since 2007.
I have a pretty good idea how these services are working,
I can develop the front end for the service you want, plus we can add some extra features like web based soft phone etc.
That being said I would like to have a little chat about your project to better understand the needs of the project
regards,