Integrate best cloud PBX to vTiger 6.10 +

We have a project to create an advanced Call Center with an HIGH voice quality using vTiger 6.10 and with free/as less cost as possible. Thes system must is going to help us: - Identify the caller which company his is calling and popup a window with all cline/caller info including all tickets and historic - Record the talk time, date, duration - Record the conversation - Count all mail coming to call center client so that we can use it to invoice the client for the emails we have answered - Integrate all email into teckens on vTiger so as it is on Zendesk So what we need help from you is to find best and cheapest PBX integration to vTiger for solving the needs described above and convert/integrate the emails into Ticken on vTiger