Why am I unable to answer calls on the Lenovo VIBE K5?

New phone, used only for a week. Lenovo VIBE K5, not able answer to the incoming calls. Able to hear incoming call ringtone, but not able to answer. After rebooting able to answer the calls only for couple of hours. After that again the same issue, not able to hear incoming call volume, not able to answer any incoming calls. Basic functionality of the phone is not working.

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