You can manually change whether your Biba presence shows you as available or busy, or leave it on Automatic.
When you manually set your presence, your contacts will see your availability as what you've chosen until you change it.
If set to Automatic, your presence will change based on whether you're on a Biba call or free, and your calendar availability (If you've synched your mobile calendar).
Keep in mind that this doesn't affect your offline status. If you're signed out of your account, or in an area where you have no service, you will show as offline, regardless of what status you have set.
Up in the top, left corner next to your presence dot, click the drop down triangle and select Available or Busy.
From the contacts page, tap your username at the top of the screen. Select Available or Busy on the Change Status screen.