Hello,
I know that I am quite new on Freelancer.com (I had my account for many years now, but it's only now that I've decided I want to start using it) but this doesn't mean I lack experience.
I have over 7 years of experience using VoIP technology and protocols (SIP, SDP, RTP, Diameter, Radius) as well as PHP for intensely used telephony applications (voicemail, rating, billing) with millions of subscribers on each platform.
If you decide that I am worthy of your attention I would love to discuss beforehand more details about your request (how big the load of the system will be, how it might interact with other equipment, how I can gather initial data, how we can test the solution proposed and others).
Thank you for your time!