Modem/router - SIP hacking

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38KB, 545x273px

Hey everyone, my ISP/Phone Company lent me the modem+router you can see in picture.

This little guy lets you call on your landline via any smartphone while you're at home, through an app provided by the ISP himself, using a SIP server which "secretly" runs inside the router. It's no big deal, for instance you can't have high-quality calls, or have more than one call at once on the line.

Anyway, some guy de-obfuscated the official app and found out how to retrieve and generate new credentials needed to login to the router's SIP server, effectively letting you call using any device running a SIP client, provided you are connected to the router via [W]LAN.
I would like to hack the router in order to let it log me in to the server even when I'm not home (thus enabling me to call and receive calls even when I'm on the other side of the world). The problem is that the server's address is a human-readable string (some.string.net) which gets translated by the router's internal DNS into its own IP (by default 192.168.1.1).
If I try to login to "username@my_external_IP_address", both from inside and outside my LAN, I get a connection timeout. Furthermore, the router doesn't let you forward a port to itself ("Provided IP Address is not within NAT range").

Do you think what I want to achieve is feasible?