Use routable IPs on eth0, phone system and WAN of SonicWall. (i.e. eth0=24.75.x.1, phone=24.75.x.2 and SonicWAN=24.75.x.3).You then can use SonicWall to NAT LAN to WAN and you can use nat overload on remining routable IP's left in the pool for fastet...
just a suggestion... why not change the nat pool to reflect the wan IP of the SonicWall, put the SonicWall on eth0 with the phone system using a small switch. Remove nat statements and add ACL for LAN access. This will leave you a way around the Soni...