Wireless infers the use of radio, this has regulatory and type approval issues associated with it.
If you want full duplex, you will need to use two frequencies with suitable spacing so that the transmitter does not desensitize the receiver.
Using two frequencies is no good if you want full duplex between several stations at the same time.
What range (distance) are you looking for?
If you want to implement a "conference" call, you will have a problem with more than on transmitter being on at the same time. The speakerphone ICs mentioned by audioguru may help here.
Good project, get your thinking cap on!
JimB