Cisco 7940 (Yea i know its prob come up before, im sorry)

Heya everyone, firstly im gonna ask that people dont jump on me for bringing this up, cus its prob been beat to death…

Iv just got myself a Cisco 7940 which iv flashed with the P0S3-08-8-00.sb2 firmware so i can use it with Asterisk, alas im having a few small issues…

The phone doesnt seem to care about authentication, the settings iv provided for line 1 were for an extension (5007) with the following details

Name : My Name
Shortname : [Blank]
Authentication Name : 5007
Authentication Password : *******
Display Name : 5007
Proxy Address :
Proxy Port : 5060

Alas, when i try and make a call, the phone will ring out at the other end (on other internal extentions), but when i pick up the dialed phone i cant hear anything spoken into the cisco one, but can hear things said from the dialed phone back to the cisco… then after 20 seconds the channel drops !

I cant call out off the cisco phone either, when dialing outbound numbers the cisco phones display shows that it is attempting to make the call, but nothing on the FreePBX operator pannel says that the call is being routed to the outbound trunk…

Additionally, although iv provided credentials for the 5007 extension to the cisco phone, the ‘Asterisk Info’ module in freePBX reports that that extension is “5007 (Unspecified) D N 0 UNKNOWN”…

Id appreciate any help on this matter, i have been looking at online guides surrounding the 79XX range of phones, under the impression that since these models have been around for some time, someone may of written a legable guide, but if their is one iv yet to find it… they all seem contradictory, or overly complecated (as with most the conversion guides from Cisco to SIP, found one which was 3 pages long about setting up two tftp servers and dhcp servers on Linux before even getting down to the actual files needed, then was pretty useless)

By the sounds of the one way sound and call being cut off it sounds like a NAT problem, but im not sure why… since this phone is connecting to a local IP (technically its going down a VPN, but that shouldnt cause any problem like this, as iv got an elmeg-IP290 working over the same configuration)