In an emergency, I would not send an SMS - I would phone. Phone calls require an instant response - texts do not.
I'd answer that text the next day, and wouldn't even be reading texts at 11.30. My line is that text and email follow the same rules as traditional post: people can post me a letter any time of day or and I will reply at my leisure.
I hope no-one will be offended if I give an alternative reading? The customer is trying to get a date for Scott to come and look at the job. Scott has replied 'later this week'. So it is understandable that on Thursday the customer chases Scott by phone and, when Scott is presumably unable to answer, the customer leaves a text, to see if Scott is free that day. Scott says he isn't available that day without giving any further information as to when he may be free, despite having previously suggested 'this week' (so if not Thursday, then Friday?).
At this point the customer wants to know if s/he is wasting time waiting for Scott and whether to should find another plumber. To find out whether Scott is even interested in doing the work, he states politely that he needs an answer in a certain timeframe. Scott now has the choice of replying and clarifying the situation or continuing to leave the customer not knowing whether to wait for Scott or not. At this point, if Scott has not replied, the customer will probably reluctantly look for another plumber.