Android :: Cannot Receive UDP In Mobile Network

I am writing a UDP client/server application. The android phone (client) can send and receive UDP packets if connected to wi-fi. When using mobile network (2G), however, it can send packets to the server, but cannot receive packets. The server does receive and send back packets in all cases. The client side code is as follows: byte[] buf = new byte[4096];DatagramPacket packet = new DatagramPacket(buf, buf.length);while (true) {packet.setData(buf); socket.receive(packet);