I just tested the same thing with Uplink logged into my LAN's Asterisk-server. Uplink was supposed to transfer a Skype-call to an internal number "1", which is my main SIP-phone. As this number dials to my local network, on this side of the NAT, just like Uplink, it eliminates any possibilities of a NAT-issue.
And the same thing happened: the SIP-phone starts to ring when Skype rings, I answer and 2 seconds later Uplink claims the number is invalid or the user is not online (Skype still keeps ringing after the Uplink-errors, until my Voicemail activates several rings later). If this was true, then why can I call the same SIP-phone with another SIP-phone, with the same internal number "1"?