Hi,
You could be having a routing issue. The voice packet could be going out on a different path and not going back to the ASA's MTA path.
there's 2 method to fix this:
1. add a static route at your router/switch. see this link for more details:http://supportwiki.cisco.com/wiki/index.php/ASA_Phone_Proxy_Troubleshooting_and_Common_Problems
2. or you upgrade your ASA to Version 8.2(1). This version fixed the MTA routing issue. But you need some minor changes on the config such as putting the MTA per interface. for me this is the best way but it's your decision and your own risk...
hth