Blackberry can receive short messages, but can't send them. It displays a red X, unknown user identity, unknown country code and short message service.

The processing method that the mobile phone cannot send short messages is as follows:

1. You can change your mobile phone to see if you can send and receive short messages.

2. If you can send and receive short messages, I suggest you plug the card back into your mobile phone and send yourself a short message to see if you can receive it.

3. If not, I suggest you check whether there is a problem with the sim card;

4. Check whether the number of the SMS center is set correctly;

5. Bring the mobile phone card to the local business hall for testing or replacement.