You will normally receive an "Out of Credit" flash SMS notification for any out-of-bundle activity such as:
Sending SMS or making phone calls (if you have a Data-Only Plan).
Trying to use Data/Voice/SMS when your data allowances have run out (on a Voice + Data Plan).
Trying to make an International call without a suitable Add-on / Bundle.
In some cases we have seen that customers would receive an "Out of Credit" notification when they haven't done any out-of-bundle activity. This is because your device has tried sending any outbound SMS messages using the FlexiFone SIM line. There can be multiple reasons for this, such as:
Sometimes your device would send these messages to verify your number in the Messages app (such as Apple iMessage on iPhones, or Google Messages on Android devices)
Often, we see customers select the incorrect line for sending outbound texts.