A cordless phone consists of a headset, handset, base station and power supply. The headset is typically connected to the handset via a 2.5 mm wired jack. The handset is usually a worn device capable of touchtone signals and low-quality voice that communications with the base station via some local radio frequency, which could eventually be Bluetooth, but is more likely to be DECT specified. The base station communicates with a typical POTS "land line" wired network, but could also be boosting a signal from a Wireless Local Loop.
Probably only DECT or 802.11g cordless phones can be fully integrated into healthy buying infrastructure and expect to become widely used by those with their own phones. Other phone-quality-audio options like GMRS or Bluetooth headsets have exterme limitations or are very expensive.