T.38 passthru from SIP to H.323

Asterisk 1.4, T.38 passthru enabled, and works great with SIP extensions.

I want to send a fax from a T.38 capable SIP extension to a T.38 capable H.323 extension.

The voice part is okay. The call is established properly. However, when I start transmitting the fax, the SIP caller sends the ReINVITE, and the H.323 endpoint hangs up.

Any ideas?

Thanks in advance