I want to implement my own VoIP client..i have seen some forum members speak about full-duplex in this context..i may sound naive..but can someone please tell me what is the need/use of full-duplex in the case of VoIP..how does it come into picture..and if i cant achieve full-duplex..does it mean i can not make my own VoIP client or what..?

As I have seen some VoIP clients already working on the mobile.if that were the case then how are they working without the support of full-duplex?