I have a 5910 handset that I can reassign extension numbers to and it will update. I can call out from that handset, but the extension number isn't visible on other handsets. I can't call into that extension. I can tried many different extension numbers and am running out of ideas.

You should recheck your extension assignment. Verify the port the phone is registering to is actually assigned the proper extension number in the ECS programming.

What does FEAT 99 display?

I don't understand what "extension number isn't visible on other handsets" means. You have DSS keys on the phones that should have one for this phone?

It is possible to have an extension without an extension number assigned, at least for digital extensions. A common error when configuring IP phones is to insert the extension number instead of the port number in the phone set-up screen.

Right - I always tell customers when doing FEAT+99 the number after EXT#. is not the extension, but the port. We never heard back but I assume FEAT 99 would just show the port and no extension.

'Isn't visible on other handsets' didn't make sense to me as handsets don't have displays, except an Omegatrek. It's very likely no extension is assigned as you said.