VirtualPC
One solution to do that is installing physical machines and then installing the VoIP servers on it, but there was a simpler way and maan! Its kool.
http://www.microsoft.com/windows/products/winfamily/virtualpc
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Also free one - VMWARE - The future of Virtualization -www.vmware.com-
Thanks to VMware I have been able to set-up testing lab in on my 12’’ laptop! Awesome! Rite now I am running a Debian and a Fedora working perfectly with different IPs, No need to format to Linux file systems, No GRUB Boot loader or anything. Unbelievable!
VMware have a lot of options but MS one is Simpler, both are free though.