I haven't done this recently, but this should get you started. Elastix is one of Asterix clones with ready-to-use Linux distribution.
- Download Elastix 2.4.0 from http://elastix.org/index.php/en/downloads/main-distro.html;
- If you don't have spare PC, download Oracle' Virtual Box from https://www.virtualbox.org/wiki/Downloads;
- Use Elastix' ISO and install new virtual machine. Give that virtual machine full access to your network (that is, set network adapter inside the VM in Bridge mode);
- Hit Elastix' forums for help http://elastix.org/index.php/en/component/kunena/
As for SIP clients: I have used Zoiper
http://www.zoiper.com/download_list.php on both Windows and Android. I am not an iSheep, so I cannot recommend anything for iOS.