Consumer Redressal

My phone Canvas A100, was given for repair regarding the power button issue. It was under warranty and hence was submitted to the Micromax Borivali service center. The phone was repaired and given back to me. Now that there is another problem with the phone, in spite of it still being in warranty, they reject the claim stating that there is no seal on the phone. According to them, they must have sealed the phone before delivery thus preserving the warranty. I was unaware of such minuscule conditions that I should have checked. Now they would not repair the phone, in spite of being in warranty, claiming that it has been opened violating warranty. Why should the consumer suffer because of the carelessness on their part? Why would I open the phone myself (allegedly) when it still lies in the warranty period?