Are you using H.323 or SIP or MGCP?
H.323:
I would tell the ATA in USA to say that ATA in Germany is its Default VoIP Gateway. And vise versa to the one in Germany.
to dial:
h.323 id that was assigned to other phone. i.e. i164 or H.323ID
SIP:
dial:
assigned number@???.???.???.???
?=IP address
I hope this helps