Master Talk Time can be used by all your numbers in your account, so that you do not have to buy Talk Time for All your numbers separately.
If the Individual Talk Time? is finished in any of the phone numbers, the system will come and look for talk time in the Master Talk Time, and use it for sending calls to your numbers.
How to enable Master Talk Time?
Click on Profile Option
Here you will see General setting, Click on Update.
Now here you can Enable or disable talk time
If this option is disabled, All the numbers will use their individual Talk Time Option.
Individual Talk Time?