Right, so after battling with this all day and lots of people saying they figured it out but not providing the answer, here it is:
When you flash the standard SIP firmware, the device has NO settings. The reason there is so much in that firmware folder is it WILL need those files after flashing.
My problem was that I was logging into admin, and under SIP settings I was pointing it at my SIP server. DON'T DO THIS. Point it at your PumpKIN TFTP server that has all the files from the firmware download. The phone will now download about 10 different files with a config like "DT00000000".
Now the next issue you have will be that it will reboot and try and download the correct config from PumpKIN for it's actual name which seems to be the MAC.
Deny any requests where it asks to download files for its MAC (You don't have them).
If it tries to send any files, accept them.
It would have sent you a DT-000*MAC*.cfg file.
Now. Before the phone reboots (or power it off), go into the firmware folder and find "dt-000000000000-sip.cfg", make a copy of this replacing all the zeros with the mac address. You should have it from file it sent earlier.
The only settings you need to change in here are the first block.
Change:
line.1.extension="" # extension on sip server (i.e. the account login)
line.1.displayname="test"
line.1.maxcalls="1"
line.1.primary.address=""# IP of SIP server
line.1.primary.port="5060"
line.1.authentication.password=""#password of the standard SIP server
line.1.authentication.username="" # I set this the same as the username/account log in of the SIP server.
Power back on the phone. It will now start pulling lots of stuff (ringtones, everything) from the firmware file if you've done this correct. The phone will now act as standard SIP phone