Yealink T42G and EHS36 failures

[ralph]

I have multiple Yealink T42G handsets with EHS36 and CS540 plantronics headsets that intermittently fail to be able to go off hook from the headset.
I can get the headset to work again in most cases by pressing the speaker button on the handset and then pressing the headset button on the handset.
the handsets are running 29.80.193.70
I have the same problem with a T46G running 28.81.193.70
I also have the same problem with a T48G running 35.81.193.70 and that is using a jabra pro headset.
they all have the same fault.
now i can use an old ehs36 thats been around for a few years and which ever headset/handset combo i plug this old ehs36 into the fault goes away.
the distributor that i purchase the headsets and handsets off assures me i am the only person that has come across this fault.
any ideas would be appreciated?