Trixbox and ISA 2006

Hi,
I am totally new to trixbox and linux and have been given the task to investigate using VoIP at work.

I have managed to install trixbox (version 2.6.2.2) successfully and managed to get 3 extensions working successfully using 3 Grandstream 200 phones. Voicemail is now working after a few changes to DTMF settings etc. (thanks to the forum)

My problem (I'm sure this will be one of many) is that I cannot get the trixbox to talk to the internet (for updates etc., in this instance installing Sendmail to be able to email voicemail).

I have managed to point it correctly to DNS, and have created a rule on our ISA 2006 server allowing the trixbox to use all protocols outbound. Yet when I run something like yum install sendmail-cf I get a message stating 'Could not retrieve mirrorlist ....' HTTP Error 403: forbidden (The ISA Server denied the specified Uniform Resource Locator (URL). ).

Now whilst monitoring the ISA server, all it's connections seem to be allowed except one that is to destination IP 204.15.73.243 with a HTTP Method GET and URL /?release=5&arch=i386&repo=os. The thing about this line that seems different to all the other lines is that there is no protocol used (others are using a mixture of DNS and HTTP) and the client username is anonymous. Now I understand that this is obviously some authentication problem being linked to it as being anonymous, but having created an allow all rule for all users I would have thought it sorted it.

Does anyone have any clues? Any help would be greatly appreciated.