ahMAD, maY be u can try 2.4 kbps version of HVXC. this should work for you. Though zig-bee support 256 kbps but it is not possible to have such a high when you have mesh network in plaCE
Its hard to implement it as ZigBee gives only 250 kbps data rate and it is theoretical. In practical it may be less. It might be possible if you compress your voice signal but it will take more time to transmit. Do you know how to configure ZigBee?