It may help for me to first explain how the IP address is derived for outbound SIP/RTP packets generated by the NetVanta 7100. When a SIP trunk is configured we will look at the IP address for the peer configured with sip-server primary command. We then use the route table to figure out what egress interface is used to reach that peer IP address. Then we will look at the egress interface used, and see what is configured for the media-gateway ip command on that interface. That setting (primary, secondary, or loopback) will determine what we use as the source IP address for outbound SIP/RTP packets generated by the NetVanta 7100 for the remote SIP peer.
If you want to use the same address on multiple interfaces, media-gateway ip loopback 1 should be used where the loopback 1 IP address is set to your desired address. For existing configurations, if you change the address, you will have to change anything pointing to the old address (SIP peers, registered phones, etc). It may be easiest to simply move the existing voice vlan IP address to the loopback 1 interface and then assign another open IP address in the same subnet to the voice vlan interface. I hope this helps.
Thank you Matt, this is very informative. Before receiving your answer I tested using a loopback interface and then all the phones unregistered. Your explanation clears up why that happened! I think going forward on new installs we will setup a loopback interface from the start and use that for everything, and if we ever need it to be sourced from the egress interface we'll use "ip primary" on a case by case basis.