We encourage you to use whatever device you already carry with you. The Duo Mobile app works on any phone and does not necessarily have to be Davey-issued. If there's an objection to the mobile app, we can send 2FA verification codes via text message instead.
The minimal amount of data and text messaging used by the system has no impact on most personal service plans, and the app does not give us any kind of access to track employees or read their data.
If you have no mobile device at your disposal or are otherwise unwilling to use a personal device for this purpose, we can offer a hardware token. This is a small device with a screen that displays 2FA codes at the press of a button.
Our assumption is that given the choice between carrying around a separate device for 2FA or using something that's already in your pocket, most employees would prefer the latter. I know this choice is not practical for everyone though, so tokens are available if needed.
I hope that addresses your concerns, if not, feel free to write back with additional questions.