VSX 5000 & Trixbox

Hello,
I have posted this in the Help forum HERE
but I thought someone in here might be more likely to help.

A client of mine has just purchased a Polycom VSX 5000 video conference device.

They would like to also use it as a conference phone on their phone system, as it supports SIP.
I have set up an extension for it & entered details on the Polycom system, but it seems to fail when registering.

This is from sip debug:

<--- SIP read from 192.168.159.201:5060 --->
REGISTER sip:192.168.159.98:5060 SIP/2.0
Via: SIP/2.0/UDP 192.168.159.201:5060;branch=z9hG4bK2297009112-19860576
Max-Forwards: 70
Allow: INVITE,BYE,CANCEL,ACK,INFO,PRACK,COMET,OPTIONS,SUBSCRIBE,NOTIFY,REFER,REGISTER,UPDATE
Supported: ms-forking,replaces
From: 2108 ;tag=plcm_2297009112-19860577
To:
Call-ID: 2297008112-19860575
CSeq: 1 REGISTER
Expires: 300
Contact: 2108
User-Agent: Polycom VSX 5000 (Release 8.7.1 - 04Feb2008 08.45)
Content-Length: 0
<------------->
--- (13 headers 0 lines) ---
Using latest REGISTER request as basis request
Sending to 192.168.159.201 : 5060 (NAT)

<--- Transmitting (NAT) to 192.168.159.201:5060 --->
SIP/2.0 100 Trying
Via: SIP/2.0/UDP 192.168.159.201:5060;branch=z9hG4bK2297009112-19860576;received=192.168.159.201
From: 2108 ;tag=plcm_2297009112-19860577
To:
Call-ID: 2297008112-19860575
CSeq: 1 REGISTER
User-Agent: Asterisk PBX
Allow: INVITE, ACK, CANCEL, OPTIONS, BYE, REFER, SUBSCRIBE, NOTIFY
Supported: replaces
Contact:
Content-Length: 0
<------------>

<--- Transmitting (NAT) to 192.168.159.201:5060 --->
SIP/2.0 401 Unauthorized
Via: SIP/2.0/UDP 192.168.159.201:5060;branch=z9hG4bK2297009112-19860576;received=192.168.159.201
From: 2108 ;tag=plcm_2297009112-19860577
To: ;tag=as6e5ed600
Call-ID: 2297008112-19860575
CSeq: 1 REGISTER
User-Agent: Asterisk PBX
Allow: INVITE, ACK, CANCEL, OPTIONS, BYE, REFER, SUBSCRIBE, NOTIFY
Supported: replaces
WWW-Authenticate: Digest algorithm=MD5, realm="asterisk", nonce="3430ce9b"
Content-Length: 0

I also get this under active channels:
192.168.159.201 (None) 2607008112- 00101/00001 0x0 (nothing) No Rx: REGISTER

Any ideas would be greatly appreciated.