SIP Proxy - Cisco 7905 & PIX

Is it possible to use the SIP Proxy to connect an IP phone to a server behind a PIX firewall? I am aware that there are some NAT issues with SIP, hence the need for a SIP Proxy. What I do not understand is if this issue requires the use of a SIP Proxy on both ends, the server’s end, or the phone’s endpoint. Currently my setup is I have Asterisk installed at my office behind a Cisco PIX firewall, on the other end I have a Cisco 7905G phone behind Endian firewall.

I am basically giving up trying to find an IAX2 hard phone and the current Linux-BSD VPNs with a Cisco PIX are too unstable so I would like to figure out some way of using this 7905 phone from home