NEC has a short list of tested and approved SIP trunk providers. I see they also have themselves down as supporting TIE emulation with SIP for system to system access. My question is, will other non listed trunk providers work as generic SIP? Or will the SIP lines only work with the trunk providers on the list?

Just use one from the list and you will be much better off.

I use Accessline. There are guides for about 20 providers and each one is different in some way.

Too much guessing going on if you pick a non tested one.

I looked through several of those guides and they are for the most part the same. This provider does no authentication, it is all trusted IP. I made the changes to the router to redirect UDP port 5060 to the DSX. Configured the port settings for the SIP provider. Added my SIP lines with the proper provider selected. But still I cannot get any registrations. And the manual does not even reference the memory blocks involved. Seems not a lot of DSX and SIP info is out there.

Who is the provider?
You Can PM me with it.

Do a Wire Shark capture, call DSX tech support with the information along with the customers data base, if you don't have a tech ID you need to get one.

IF you don't have Wire Shark it's available free on the internet.
It's a great tool to have, also, remember you have to mirror the switch.