Hi, I'm also from Sri Lanka. To be frank, I have done this for my work place. We are maintaining a call center which is totally based on Asterisk. If you want to test my work I can give you our hotline number so that you can call and verify. If you want to test, please call me on 0714530558 then I'll give you the number, and if you want I'll make a demo for your particular requirement.
Since I'm just starting as a freelancer I would complete this project at the lowest cost and I don't need any advanced payment. (But seems like www.freelancer.com does not support that). In addition to that I can teach you how to configure it by your self.
To install AsteriskNOW you need to have a PC. (To testing purposes and learning purposes we can use a Virtual Machine so that there is no need of a separate PC)
I think I am the best option for you.
I have experience with SIP trunks, Asterisk, FreePBX, Elastrix, Freeswitch and FusionPBX
Thanks