VVX410 w APP-51 EHS and CS540 not communicating

We have a FreePBX setup with commercial EPM, most phones are polycom VVX410 with firmware 1.09. EPM is at

Decided to connect a Plantronics CS540 headset to one phone using the APP-51 EHS adapter, like I have on other hosted systems but no matter what settings I use I cannot change the phone Hookswitch mode to Plantronics EHS. If i select it, the phone reboots and then comes back at the default EHS mode.

It seems to be a limitation in freepbx, as like I said I have other systems using the exact phones, cables, headset.

Any help would be much appreciated.

Has anyone else had an issue with enabling Plantronics EHS hookswitch mode on a VVX-410 or similar Polycom phones?

I cant see why this feature isnt working on all of the phones.