A couple weeks ago I came across one of my HPBX SIP customers with a 4430 that doesn't have SIP Transparent Proxy enabled. Using the document "Configuring the SIP Proxy in AOS", I added these lines to the config:
no ip firewall alg sip
sip proxy transparent
Everything appeared to be fine but the SIP endpoints continued to be seen as a NAT'd devices by my SBC, when I took a packet capture I can see that the source IP was that of the public side of the NetVanta and the Via was still reflecting the private DHCP pool.
There are two lines in the customers current config that I am not familiar with that are not present in my lab R11.10.7.HA.E, they are:
voice feature-mode network
voice forward-mode network
Could these lines be preventing the unit from enabling TP? And is TP fully supported in R10.9.6.V?
Do you have sip turned on globally? The command sip or ip sip in current firmware may be needed. Also, I'd leave the ip firewall alg sip enabled.
Note that the transparent proxy only works if the phones are using UDP, not TCP.
Transparent proxy is supported in R10.9.6 but it's ancient code. I'd recommend upgrading to the latest extended maintenance release.
The voice feature-mode network and voice forward-mode network commands determine whether forwarding and features (DND, etc.) are handled locally or passed to the network. The other option is local. Network is the default and won't show with "show run" in current firmware unless you look at the verbose configuration.
Re-enable ip firewall alg sip
Enable sip or ip sip globally
Upgrade to the current extended maintenance release.