Headphone Jack Changes Volume Automatically

Hi,

I am using a Latitude 3540 and trying to get a conference phone working through the Headphone port. The phone has a mic and speaker cable that go into an adaptor and then into the headphone port on the laptop. As soon as its connected the volume slowly decreases on its own.

We have tested the same setup but using a Lattitude E6330 and it works fine.

I hve fully updated all drivers.

Any ideas?

Thanks

Ross