We're currently using the PushRegistry API and listening on port 5012, which works fine when the server and the phone is on the same network.
 
The problem is, we need to be able to push a message to a mobile phone from our web server, but the phone is on a private network and therefore not visible to our server. Is it possible to use JTAPI / XML to send data to port 5012?  Suggestions?

With a UC5x0 and CME, the Cisco IP phones may be on a private NAT network behind the UC5x0 and not directly accessible to our application server. Currently, this is not a problem for us, since, with the UCXSI SDK, we can send an XSI request which the UC5x0 relays to the phone.
 
What Russell is asking is - is there any way of doing something similar for proxying communication to a midlet application running on a Cisco IP phone?

Although it's not technically using XSI, if the idle url is pointing to a midlet, the phone will pull in and autostart the midlet when the idle url timer fires.
 
Same with the PTT button, if that button is mapped to the 1st service in the list of services that the
phone is subscribed to, and that service happens to be a midlet, then when the PTT button is pressed
the midlet will be auto started.
 
I don't know if there is a CiscoIPPhone Execute item that could invoke either of these two options, but I
do know that the manual invocation of the idle url and the PTT button does work.