Why can't I send short messages from abroad sometimes?

Take Jiangsu Unicom as an example: First of all, I need to remind you that the way you send text messages abroad is exactly the same as that in China. Secondly, you need to check whether the SMS center is set correctly. Usually, China Unicom will set the SMS center number on the user's USIM card or 3G customized mobile phone. Thirdly, when sending short messages to international, Taiwan, Hong Kong and Macao users, the sending address is "00 opposite country code opposite number"; Or "+opposite country code opposite number". In some countries and regions, the network only supports "00" sending mode, please note here. If you need help, please call China Unicom customer service hotline 100 10.