We have a Mitel 5330 IP at work and it's asking for a PIN to be entered when it is first booted up, ie, enter the PIN then the hold button. This is as far as we can get. This is the only model of the phone we have, the rest are 5212 IP. Would there be a default PIN for these phones? If not, how would we go about sorting this phone out?

You must contact your Mitel phone system administrator. This is a complex question that could have many answers. This is not a 5330 phone question.
Good Luck

First, you have to unplug the Ethernet cable from the phone to turn it off. Second, remove the receiver from the hook. Third, hold the #6 on the keypad as you plug in/turn the phone back on until you get a prompt on the screen. Follow the prompts using * and # to signify yes or no respectively and you should come across an option to perform a factory reset. I hope this helps.

In system options you will see a registration code enter that code and then the ext number of the phone then hit hold. This will register you phone. You will need to have a ext programmed in users and devices.

Pick groups are programmed??

Blue key and you should see the people button

Problem with network or maybe cable. Phone can not connect to Mitel phone system.

I had this problem as well. This is a configuration that needs to be added to your DHCP server to enable the mitel system to obtain an iP address. If you reboot the phone and hold the down arrow it should bring you to a screen you can manually enter the IP address of the network you are useing.

Option 128 tells the phone where the phone system is. A linksys router isn't capable of giving this option out. Where is the telephone system? Is this a teleworker phone? if TW phone, press and hold the 7 key while booting the phone, and type the teleworker server IP address.
I recommend you contact your administrator or system vendor for guidance.

I don't think your Class of Service is set to allow the Call Forwarding to be enabled. You should contact your vendor for support.

My guess is that your DHCP server isn't set up properly, or someone has connected another Router to your network.