According to the terms and conditions it appears you have 60 days to restore your service and you can keep your number:
Copied from Terms of Service:
"If you fail to make payment when due, your Service will be suspended and/or cancelled. By making a payment, you will have access to Service for the designated period of time. If your account is deactivated for non-payment, it will be suspended for 60 days. If payment is not made within the 60 day period, it will be cancelled. When your account is cancelled, we will re-assign the phone number associated with your account, and that phone number may no longer be available to you."
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