How do I know when I should trigger the Manual Top-up for 1511EXPRESS account?

ZONE Telecom > Consumer > 1511EXPRESS
Once your account balance reaches $5 and below, you will be prompted via SMS to top-up your account.
To top-up your 1511EXPRESS account, just follow the instructions on your SMS alert; ie. SMS topup<space>registered tel. no. and send it to 81231511. 
Please contact our Customer Care Hotline: 6259 9453 if you need assistance.