Unexpected charges on iPhone 5 | Phone/Device Issues and Features | Consumer forum

I recently logged into my 7-Eleven SpeakOut account online to top up my number, only to find my account balance had been significantly drained, even though I haven't been using my phone to send texts or make calls for the past several months.

When I called SpeakOut, they told me I have been repeatedly charged $0.15 for the past three months for short codes (149988) sent from the FaceTime app on my device. However, FaceTime and iMessages have always been disabled in Settings on my iPhone. After monitoring my account balance closely, I realize I am getting charged $0.15 whenever I turn off/on my iPhone.

When I called Apple, they said FaceTime activation was failing but they weren't able to offer a resolution to the problem. Has anyone experienced this issue and know how to prevent this short code from being sent? I have an iPhone 5 with iOS 10.3.3 (which is the last supported version). I don't use FaceTime or iMessage, so I would prefer to get rid of them completely if that solves the problem, but I don't know how to do that.