Duo provides several options, including a smartphone or tablet app, phone calls to landlines or cell phones, and text messages. Instructions on enrolling in Duo are available at http://its.umich.edu/two-factor-authentication.
The following is an example of what opening a session on Flux might look like.
$ ssh flux-login Password: Duo two-factor login for uniqname Enter a passcode or select one of the following options: 1. Duo Push to XXX-XXX-1810 2. Phone call to XXX-XXX-1810 3. SMS passcodes to XXX-XXX-1810 Passcode or option (1-3): 1 Success. Logging you in... uniqname@flux-login ~$