Good morning. I have recently tackled this undertaking and must say that it is not as straight forward as it may first appear. If you are attempting to have the PBX manager integrated with Vtiger 6.1 and Asterisk there is heavy modification that must be done to compiled source. Right now I have a floor with 24 seats running on this system with the capability of growing to approx 70-80.
I would question any other devs that claim that they may be able to do this on a cheaper scale, I highly doubt there will be results as my team and I have spent the past 3 months on this exacting problem. I attempted to hire a VTiger expert to get this system that I currently have fast tracked and they could not even accomplish what we have.
The team that I work with is extremely talented, speaks perfect English (I paid for the test to bid on this) and each of us have communication skills for different situations. Keep in mind that VTiger 6.1 is still technically beta and not suggested for production environments. I have broke this suggestion and have only a few minor bugs such as reporting not working on a schedule.
There are also many other variables that were not addressed in the request such as desired endpoints, internal access, external access, provider, hardware.