Customer has UC540 and SPA525G2 phones. The "phone load" on the UC540 installs firmware 7.5.2a on the phones. The customer wants to use Plantronics headsets with EHS (Electronic Hook Switch) and there is apparently a bug in 7.5.2a that prevents EHS from working. I see 7.5.4 firmware says EHS has been fixed.
So is there a way to add the 7.5.4 onto the UC540 so the phones will downlod it and upgrade themselves? Is there any danger of the phones doing this upgrade over the VPN?
You can get the firmware from Cisco's website (Support -> Downloads -> SPA525G2, etc)
When you have the firmware on hand, open CCA and drag/drop the fimware as suggested by Brian Rapier.
As far as 7.5.4 fixing the EHS problem, I confirm that in our case it did indeed solve 3 clients with that very same issue as you are having with Plantronics. So I think you can be confident the issue will be fixed after that.
BUT, you need to do some tweeking on the phone themselves after the phones are on version 7.5.4
First, you need to enable Web editing by physically changing that on the phone. Somewhere in the settings of the phone.
Then, you need to access the website of every phone that will be using a headset and under the "Voice Menu", you go under the User tab.
You'll have a "Electronic HookSwich Control" dropdown that you will change to yes.
By the way, as of June of this year, there is a new firmware version 7.5.5 . I haven't tested it yet though so I cannot comment on that version.