I have a Galaxy Note 8 and I receive a triangle with an exclamation mark inside after sending some messages. The message sends as should but it comes back with the triangle alert. Can that be explained? There is no need to turn off and restart, it is there. There is no need to hold the alert message it gives the same options that are supportive of the problem. . . and as I stated it successfully sent.
Actually the triangle symbol means there was an issue with sending the message. It may have been sent from your phone, but it wasn't received on the other end. You can try to resend it usually by long holding the individual messages and selecting the resend option in the menu dialog box that pops up.
I'm most definitely NOT a VZW employee. If a post answered your question, please mark it as the answer.