When you add credit to your account (using the "Get Credit menu or our Online Top-Up feature) it is directly added to your account balance. All credit in your account balance is fully accessible to all of your prepaid numbers by default.
Should you run out of credit on your 'Phone Number Balance' then your number will automatically begin taking credit from your account balance instead. If you go into your number menu and select an active Prepaid (non-Unlimited Plan subscription number), you will see your amount of SMS/minutes listed from 'Phone Number Balance' as seen in this screenshot, showing a number with no number balance left.
If you swipe that field to the right or left you will be able to see the amount of SMS/minutes you have calculated with your 'Phone Number Balance Using Account Balance' as seen below
❗NOTE: On the pay-as-you-go '30 Day Worldwide' plan, you will only see 'Phone Number Balance Using Account Balance'. This is because all calls/texts on this package type use Account Balance only.