Using an Android phone as a cordless headset I know there are some cordless phones which allow adding Android phones as additional headsets (e.g. http://shop.panasonic.com/shop/model/KX-PRW130W or http://shop.panasonic.com/shop/model/KX-PRX120W). These devices include both a headset and a base receiver. I already have an Android phones, so I only need to a device to transmit calls to my cell phone via WiFi (the base). Is there such device available?
In the links you provided in your question, are the devices that are required.  They have that feature built into the hardware.