Your Esendex SMS Zap will only check for a trigger when the Zap switched on.
Once active the syncing interval, which is the frequency that your Zaps will check for new data, are dependant on the type of Zapier plan that you are on. The current interval for checking the trigger app is between 5 and 15 minutes.
This means that once an event takes place which triggers the Zap the text message won’t send immediately – unless your Zap has an instant trigger. You can find more information on the types of plan and syncing intervals in the Zapier guide.