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The reminders are not guaranteed. You can set up scheduled top ups which are guaranteed to happen. Or set a date in your calendar as an additional reminder.
Apologies for the delay in responding, I'm afraid cookie is correct, notification text messages such as this are not guaranteed every month so we would always advise our customer to view their top up offer information through the My Vodafone app or you can check your offer end date by free texting BAL to 50233.