I have an old Plantronics CS70 NC wireless headset. The battery is
dying. When I checked the owners manual, I was shocked (Shocked, I say)
to discover that the headset has to be mailed back to Plantronics to
have the battery replaced.

An Internet search turned up several companies that sell replacement
batteries, biut none of them have instructions. The problem is that I
cannot figure out how to open the battery cover. It has a little
Phillips screw, which I removed, but that doesn't seem to hold the cover
on.

There is another round "cap" that appears to require a special tool.
I've uploaded several photos:

https://www.dropbox.com/sh/zs29tax9cflnu42/64Yz3HrNJL

I called Plantronics. The tech told me that that unit is no longer
supported and I need to upgrade to the newer model. I told him that the
device works fine. It's just that the battery is dying. He was not
sympathetic. I asked him if he could suggest a way that I could remove
that cap. He first said that he is not allowed to give out that
information and then that he does not have that information.

One website said that the battery is soldered in place.

This is the last Plantronics product I will ever buy.

I tried grabbing the little edges with needlenose pliers and tweezers.
No joy.

It looks like it is threaded. I'm wondering whether I could epoxy the
head of a Phillips screw to the cap and use that to unscrew it.

Any better ideas?

I don't care too much if I damage it. It's almost useless as it is and
Plantronics will not repair it.