Subaccounts are not able to purchase their own balance but are either:

• Assign the balance that they use from the main account as a budget.
• Share the balance available on the main account.

When you add a Subaccount (Workspace → Subaccounts → Add Subaccount), you’ll be asked to choose between those two options.

The moment the assigned budget runs out, or the main account’s shared budget is used, the Subaccount won’t have any more balance to continue sending messages. That’s why maintenance will be needed to ensure sufficient balance is available for each to keep sending messages.