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Dicono di controllare le impostazione del router.

NON SERVE

Invece andare a:

http://www.sipura.com/Documents/faq/Section_3.html

10: My SPA is behind NAT, I can call by IP and the other unit rings, but no voice ?
A: Line tab setting; change "Nat Mapping Enable" to yes
SIP tab; change "Substitute VIA Addr" to yes, and "EXT IP" to your router's WAN IP.
Alternatively, you can try to use STUN. (STUN will not work w/ symetric NAT router)
Or, You can port forward SIP ports as defined in Section 1 -5

ciao