Ipone4 mobile phone 186, open international roaming. In Germany, people can communicate with each other and send messages to China, but they can't get messages. Why?

Unicom's international roaming is very water. I used to make foreign phone calls, saying that I couldn't get text messages when my phone was turned off. It was because there was a problem with the roaming agreement between China Unicom and the local operator, and he switched packets after switching networks many times, resulting in an error. Mobile will also have this situation, but much less. You can call Unicom to complain, and they will find the problem and fix the BUG.