Hi Rachel -
What type of phone are you using and which mobile provider?
It sounds like you're not alone. This older discussion includes others who have not been able to resolve the SMS issue.
Have you contacted Canvas Support (How do I get help with Canvas as a student?)? Have they been able to share anything with you?
I would recommend you to use another telephone number. But if you do not have any or you simply do not trust to your friends/parents... Then try to use https://grizzlysms.com/uber service. It helped me a week ago to set up a temporary phone number for uber as I could not receive SMS code on my number here in Netherlands.
Hope your problem will be resolved soon.