My mobile phone, iPhone4s Telecom, used to be fine, but suddenly I can't send a message to a number recently. Others are completely normal, and so am I.

This reason is mainly because the receiver's mobile phone inadvertently blacklisted the sender's mobile phone number.

If the other party is Apple, you can make the following settings:

First, enter the settings;

Second, click on the "phone" below;

Third, click the "blacklist" under the mobile phone;

4. Click Edit, then find the sender's mobile phone number, and click "one" before the sender's mobile phone number to delete the blacklist.