IPhone 4S sleep button issue?

The sleep button on my iPhone 4S is broken and no longer pushes in. I was wanting to know if anyone could tell me if this is an underlying issue with this iPhone or if it my fault. I'm arguing with the company who sold me the phone that it is a common and inherent fault and wanted to get something to back this up. If anyone could tell me what causes this and if it's a design issue I'd be very grateful.

Added (1). The on/off button at the top

Where is sleep button?

Keep clicking it

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