The sending job only runs when your balance is greater than the per-message rate; otherwise it stops with "NO CREDITS" and queued messages stay in History > Sending until you top up (they are not lost). Incoming texts are only stored if the balance covers one message — with an empty wallet, replies from customers are dropped and will not appear in Chat. On Compose, the confirmation step shows Balance After in red and an Insufficient balance — add credits to continue box with quick Add $50 / $75 / $100 (+3.5%) buttons; the CONFIRM button is hidden until the balance covers the campaign. Turning on Auto Recharge prevents this.