You might have occasions when you had too many customers wanting your service at the same time. Maybe you advertised an exclusive deal for the new gadget you now have in store, or maybe too many people want to get their mobile device serviced, etc. you have a long line of potential customers, and you don’t want to lose them or make them angry by having them wait in line.
Tell them approximately how long it will take, ask for their mobile number, and tell them you will call them or send a text message when they are next in line (give them five minutes to get there). They now have time to get some coffee, go shopping, etc. if you want to be even nicer, offer them a free coffee or cake coupon for a nearby place. This way the waiting time will not only pass faster, but they’ll have a much better experience, and it’s all connected to you. You have turned a potentially bad experience to both of your advantages.