Phone-Proxy, ASA 8.0(4) & EIGRP = BAD
After battling with getting Phone Proxy working on an ASA for a bit of time I finally tracked down the problem. It all started when I tried to turn on phone-proxy on an ASA5505 running 8.0(4). I started getting very strange debug outputs with very strange TFTP messages. What I noticed is that after random intervals of time the ASA stopped passing traffic. What was happening was the ASA was running out of 80 byte blocks. After working with TAC we discovered the issue was relating to the ASA running EIGRP. As soon as I removed EIGRP from the ASA it stopped dying and phone-proxy magically started working.
If you need EIGRP on your ASA open a TAC case and they will send you an interim release with a fixed. I believe anything past 8.0(4)12 will do it. For more details check Bug ID CSCsv89678 (CCO Required).