Safety concerns about sensitive data

Hello everyone.
So I have a website built with wordpress and I have to create a page that customers can go in it and enter their phone serial numbers and get an activation key sent to them via SMS. I have an excel sheet that contains all the IMEIs and their activation keys. Since these information are very sensitive, I am hesitant about how to do this implementation. Should I directly store the excel sheet in my host and make wordpress match the data and send the code? What are the most common security breaches in this approach and what is the best course of action to take for this kind of thing?