iMessage trying to send without wifi or data

Using my iPhone 5S (iOS 9.3.1), iMessage has been finicky since setup of the phone. When my phone isn't connected to wifi and when data is turned off, the phone still wants to send an iMessage rather than a text. I have to force send as a text. Typically, I receive a message about 5 minutes later saying "Message to (contact) failed to send" when in fact it did send and go through to the recipient. My phone will then resend the text, causing the person I'm contacting to receive double messages. The only fix I've found is if I force send a text 3 times back-to-back, the phone will finally default to sending as a text. After calling my carrier about the issue, I reset iMessage and the phone itself, but the issue persists.