Outboundproxy does not work on reinvite


I have question about outboundproxy option.
I have set outboundproxy in sip.conf under general section. It works well - Invites and other packets are sent through Proxy on outbound calls.
However, when I am placing call to T38 fax device:
Invite goes through proxy to T38 fax device;
T38 fax device responds with T38 reinvite, and here starts the problem: Asterisk sends Trying no to Proxy IP anymore, but to T38 fax device directly.
Are there any options to make Asterisk respond through Proxy in such case? Or maybe it depends on Headers used on T38 reinvite which comes from Proxy to Asterisk?

In needs to say, that it is same story with any re-invite, not only with T38.