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.