I have done some more testing on this set up.
The user was a hot desk user, when he logged off his handset and it was just the base extn, the button on the headset next to the earpeice can be used to answer and clear down calls. That's the switch on the headset not the headset key on the handset that needs to be programmed up.
When the user logged in, the button doens't work for picking up calls and clearing down. So i know the headset and the module and the handset all work, it looks like a prob with the user account.
I have set him in the same class of service as a user I know that can use his headset to answer/clear down calls and removed him as a hot desk user, but still he can't use the button on the headset to answer/clear down calls.
I have cleared his extn using #*#* in case it was a user set feature code that had been put in, but for the life of me I can't work out why he cant use the button to pick up calls.
I blame the user!!!