Andre,
I spent hours on this exact configuration and was never successful in getting it to work. I tried every possible scenario with both PIX and ASA firewalls. I even opened a TAC case and they said it should work, but we never came to a resolution (shame on me for not sticking with the case).
As an alternative solution, I installed Cisco Unified Phone Proxy which takes all the pain out of NAT, Firewalling, VPN, etc. and makes remote endpoints much easier to manage and configure.
Thank you,
Jon
