Have you fought the fight to setup ASA phone-proxy (back in the day) or VPN Phone on the ASA? While VPN Phone was a solid solution, it could be a bear to setup and troubleshoot.
So, file this in the how did I miss this folder, but CUBE 10.0 (IOS 15.3(3)M) on the ISRG2 and ASR supports line-side SIP proxy for all of the current generations of phones (69xx, 79xx, 89xx and 99xx). So CUBE can do the dirty work for you.
I’ll set it up soon and document my experience with it once I get my CollabEdge guide out, but here is more information: