disturbance created by talking in cell phone

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When I am working in my office i have been facing this problem. As my job is direct customer related and so many customers are coming to my office. As far as my profession concerned all are important to me. While doing work of some customer the others have got call over their cell and they talking through it. But i am loosing my concentration when some one talking over phone loudly. Please help me how i manage this problem without hurting any of my customers. I got vexed out with this problem these days.