Can recieve SMS texts, but can not send them

I am able to send and receive iMessages completely fine. I am able to receive SMS texts without a problem. However, I am not able to send SMS texts at all. Instead, I get message delivery failure errors.

I have gone to my cellphone network provider. They said it was not a problem with the SIM card, as they switched my SIM card to another phone, and the card/phone was able to send SMS messages.

I attempted to download the new iOS thinking it might be a problem with the operating system. Didn't work.

I attempted to reset network settings. Still didn't work.

I backuped the content, then erased all content, and reset my entire phone. Still didn't work.

At a loss of what to do.