It looks like this requires the staff to download software on their private phones for a work reason.
I have the same issue at work, they want us to use lastpass but the ONLY way requires that i download an app to my Private phone (I have no work phone)
If they want to provide me with a works phone then i will use that to download the app but why should I use my PRIVATE phone for work?
After all, I do not always remember to carry my phone around with me!
Anyway not everyone has a mobile phone and even if they do does not mean they are running a compatable OS (Android/IOS) seems to be the only options, but windows mobile etc.