Ok, I resolved this problem by reverting back to As soon as the phone finished booting with the firmware, I heard the CS540 base click active. I pushed the button on the headset to deactivate it and I knew it was going to work before i even picked it up to test.

This does not explain why the EHS previously worked with

After confirming everything working correctly, I upgraded to The headset no longer worked correctly. I downgraded back to and it worked correctly again.

This is a problem with the new firmware for the phone and obviously not a problem with the EHS firmware.

Email sent to yealink support. Hopefully this will be resolved in future firmware.