Mitel 5224, 5324 and 5330 SIP Setup Assistance Required

[Cirrus]

Hi All,
The company I used to work for is closing down the site I was based at and as such they have decided they can no longer make use of the telephony equipment they had and have kindly donated around 40 handsets to me.
These handsets are a mixture of Mitel 5020, 5212, 5220, 5224, 5324 and 5330's.
I have only ever used these handsets when they were attached to a Mitel 3300 PBX so I'm new to SIP but to make use of some of them, I would like to set them (those that are compatible) to connect to a SIP services.
I have done some research and found how to convert them to SIP Mode but I haven't been able to get much further.
I get the message "Unknown File Type, Downloading... 1" on the display of each handset when they boot up.
I managed to find the IP address of one of them and can get onto the web console and I can see that the SIP firmware is 04.00.00.07 which is really old so I'm wondering if it's just because the firmware needs updating?
I managed to find firmware version 8.0.0.4 but it doesn't seem to let me upgrade from 04 to 08 (or I'm doing something wrong) but even this version is quite old (2009).
I went through the settings on the web page and I can actually receive calls on the phone now through the SIP service provider but the error on screen is still the same.
Please could you help me, I'm new to this so please keep it simple in your reply if possible.
Thanks in advance
Cirrus

[Cirrus]

UPDATE:

The Good News 
After hours of messing around, I have managed to get a 5224 handset to upgrade firmware and have successfully registered with sipgate and I can make and receive calls.

The Bad News 
1 - I'm not 100% sure how I managed to get it to work

2 - I have been testing for some time and receiving call is not reliable, i.e. If I make calls to the SIP number, some of them ring the handset and some go straight to voice mail. 
I have noticed that after a reboot it seems to work fine for a while and then I get the voicemail after maybe 15 minutes, so I'm thinking it's a timeout issue of some kind, can anybody help with this and suggest what settings I should try changing as there are so many I'm a little out of my depth.

Thanks
Cirrus

[ralph]

Not that I can help in anyway but I'd be curious to know what type of SIP GW you're connecting to.

Ralph

[Cirrus]

I don't know what there setup is I'm just pointing straight to sipgate.co.uk
 

[Cirrus]

The intermittent service was related to using a Virgin Media Connection with the Superhub 2 and not setting in relation to getting a working service on sipgate.

[brettuk]

Any chance you can remember how you did it? I am setting up 3x 5212 mitel phones and 2 of the 3 phones don't have the web page enabled so I cant get the sipgate info into the telephones

So stressed!!! 

[MarcB]

For Mitel 5312 IP Phone:
Press and hold *7 whilst rebooting the Phone
Hold until you get confirmation that the Phone is now in SIP mode

With the Settings key, navigate to ADMIN TOOLS/Network Settings.  (Password = 5312)
Note the PHONE IP ADDRESS

Log in to the web-interface with username = admin, password = 5312

Now you can set the User credentials and perform Registration.