I've got a client who's currently using Grandstream GXP-2110 phones and their previous Plantronics CS55 headsets. If I plug the headset into the headset jack of the phone, the headset button just toggles the headset for an existing call, but does not answer/end calls. So for an existing call, pressing the headset button will transfer the call from the handset to headset and back, but pressing the button will not answer an incoming call. The current setup has the CS55 connected to the handset jack of the GXP-2110 and the handset connected to the CS55. And a lifter is used for answering calls. Are there headsets available that are compatible with the GXP-2110 and that can answer calls without the use of a lifter?