[Asterisk-Users] How to register two SIP phones ( e.g. Windows Messenger) from different subnet to *

[Luki]

You you can reach the * server from both location, it's straight
forward, standard registration; from your post I understand that this
is NOT possible as * is in the XXX.XXX.25 subnet and phone 2 is on
XXX.XXX.15.* which is completely isolated).

So given your current config, no, you can't do it. Not unless you
somehow can route traffic from XXX.XXX.15.* to XXX.XXX.25.* (and
back). You can, however, more the * server into a third subnet that is
reachable by both phones (even if the phone cannot reach each other
directly).

--Luki