What about Asterisk?

Consequently, all moves to ensure that the transmission of voice data using local computer networks and the Internet. That means when the development of network infrastructure that does not contain analog devices, server, IP PBX is a conventional system unit (best server in the rack), with one or more network adapters. Itself eliminates the need for expensive specialized equipment - for example, when expanding the network instead of an expensive expansion cards (to increase the ports on the PBX) uses a regular network switch.
See more: Asterisk, Solutions for ISP's, Call Center Software

Load Capacity Asterisk is extremely high: up to 30-40 analog channels for a single-core processor, 2 GHz, and digital - much more. As the exhaustion of computing power can easily increase the performance of hardware. Asterisk runs on Linux operating system or FreeBSD. There are also inexpensive hardware solutions based on Asterisk, able to serve small offices.