iphone4. Can't send text messages

Reasons why iPhone 4 cannot send text messages:

1: The cellular network signal is poor and the connection is unstable.

2: There is an error in the system network settings.

It is recommended to restore network settings.

The operation is:

1. Click "Settings" on the main interface of the machine and turn on "Airplane Mode".

2. Open "General" on the "Settings" interface and select "Restore":

3. Select "Restore Network Settings".