There are 3 options that can be setup as a preferred approval method. Depending on the method set, you will have to complete the approval before you can complete the sign in to the account.

Via Authenticator App: Android / iOS

Experience: When approval is required a notification will appear your phone.

TIP: Mobile Mentor recommends this as the most effective option.

Note, some companies recommend or restrict employees to one of the other methods below

Via Text Message 

Experience: A text containing a 6 digit code will be sent to the number you register, enter this number in the prompt shown on screen

Via Phone Call

Experience: You will receive a phone call, once answered press the # (Pound / Hash Key) to approve