BT Shop has this BUDGET VOIP HANDSET for just £5.99 inc FREE delivery.
Edit: use attached voucher (code: BTBBSAVE) to get this for £5.09 delivered.
Handset for calls with BT Communicator. Plug in your speakers and listen to audio from your PC when not making calls. Switches to handset mode when lifted from cradle. Plugs into the ear and mic sockets on your PC sound card.
Note: The description says that this handset is for BT Communicator but I think this can be used with any VOIP program. Correct me if I'm wrong...