The most common reason to get a message saying you don't have enough credits is that someone is trying to send an ENHANCED message, which costs (2) credits per person to send.

These are great because they allow you to send text longer messages or messages with pictures. 

When you're about to send an enhanced message, the whole screen turns blue and there is an alert at the top. Otherwise, you have to keep it below 160 characters with no image to send as a standard message. 

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