Hello, problem solved. The user has changed the APN on his phone, activating WAP profile. In such way, all the communications are "bridged" to the real internet by the WAP access point. Changing back the APN to the "normal internet" solved the problem. All the packets are sent and received without any problem, regardless the cell that the phone connects to. Note that if you change the APN to the "normal internet" option (in my specific case, the internet APN is "ibox.tim.it", while the wap APN is "wap.tim.it") the phone will revert automatically to the WAP one, probabaly 'cause if you purchase a SIM card for "smartphone use" it defaults to WAP connections. Using WAP gives you more security in web navigation, but less power to send or receive packets not specific to web navigation. To overcome this fallback to WAP problem, simply create a new APN entry ; call it, for example, "MyApn" and put in it the "internet apn" data (as mentioned before, in my specific case, "ibox.tim.it"). With this new entry, the phone will NOT switch back to WAP APN and a program running with UDP packets will work again.