SPA-921 remote extension problem

[w1zard]

I'm using a spa-921 hardphone at home and i'm trying to connect it to my office.... but I'm facing "ip problem"... my extension get's my private ip instead of my outside ip... [email protected], i tried to configure a STUN server with no success, keep getting the same ip.
I have
STUN enable -> yes
STUN server -> stun.3cx.com
Substitute VIA Addr -> Yes
Send Resp To Src Port -> Yes
NAT Mapping Enable -> Yes
NAT Keep Alive Enable ->Yes

..is something missing...
...help