Great question. By default, the TA 900 won't do anything special to international calls originating from the PBX. You can enable the ADTRAN to look for the international call type by configuring an ISDN number template. The template would define the call type, the called party string to match, and the prefix to add.
In your case, the template would look something like this:
isdn-number-template 1 prefix 011 international X$
You could also add E.164 formatting by using the following command:
voice international-prefix abbreviated
That should do it!
I went ahead and marked this post as "assumed answered". Feel free to mark any correct or helpful answers from this post. If you still need assistance with this issue I would be more than happy to help, just let me know in a reply.