I’m on the Ultimate plan and I don’t know why I ran out of messages.
Please check the length of your message template. The app might use up more than one text message per reminder. The length of your message is dynamic and depends on the text and fields you’re using. The standard length of a text message is 160 characters, so if your message is longer than this, please consider shortening it.
Using Unicode characters (Emojis or special characters) also has an impact on message consumption. It will shorten your character space to 70 characters thus consuming twice as many messages per reminder.
Please check these by going to Menu → Settings → Messages → Tap on the message → Template.
To purchase more messages please go to Menu → Settings → User account, and then tap “Buy more text messages”.