Well, DrPb x's answer doesn't really answer the original question. A 3rd party SIP license gets installed on the NEC phone system and allows one to use a 3rd party SIP phone (one that wasn't made by NEC) to be use on the NEC phone system.

However, Tom is trying to make his NEC DT730 to work with Asterisk phone system and thus needs to change the firmware on the DT730 for it to support open SIP standard, not just NEC's.

I also wanted to do this some time ago, but couldn't find the said firmware and it wasn't high priority, so I never pursued. But I'm still interested, so if someone here can come up with the firmware...