FRANKFURT — Plantronics has developed a Bluetooth headset that can tell whether or not it is lodged in an ear. That means users can accept calls simply by plugging the Voyager Pro HD into their ear. Taking it off means the phone call is then picked up by the linked mobile phone. The company is also introducing two new apps for iOS and Android. MyHeadset includes instructions and a variety of tools for testing the gadget. InstantMeetings, also available for the Blackberry, allows users to set up conference calls. The Voyager Pro HD sells for 100 euros (US$138 dollars).