Does anyone know why Pantech a760s messages can't be sent, but they can't be received or sent?

Mine can be sent. You can set the information center number in SMS settings. If not, you can delete the local information, download the enhanced version of GO SMS to replace it, and then install it in the RE Manager system /app. After installation, set the parameters in the settings of SMS/MMS in GO SMS Enhanced Edition, so that the sending and receiving of SMS and MMS can't be recognized by the operator except the instruction to check the traffic.