There is no pause functionality in focus booster because it is an app to implement the Pomodoro Technique.
The pomodoro technique calls for all distractions to be removed while you are in a session, so you can focus fully on the task at hand. We want to be true to the technique because we know it really works! So, phone calls, emails, toilet breaks, social media, colleagues and other interruptions should be dealt with in your break or a dedicated focus booster session throughout the day.
That said, focus booster does give you the option of saving an incomplete session to your timesheets if you need to keep a record of your time, say for billing purposes or reporting to a manager. Whether you choose to save the session or not, the timer will be reset to start a fresh session in the next round.