Arduino Cellular Shield: UDP packets reception and TCP listening sockets

With the Sparkfun Cellular Shield equipped with SM5100B-D GSM/GPRS data module, I successfully tested UDP data sending to an outside UDP client, but the test fails to receive UDP packets from the client, even with a precisely defined local port with AT command AT+SDATACONF. All the provided handbooks didn't provide details on UDP sockets and also for TCP listening sockets. Actually the data module seems unusable for these purposes. Do you have some suggestions? Thank you in advance.