Unhappy [Q]I need precious Help

My HTC Explorer is two years old and everything was working fine, suddenly i got an issue. whenever somebody call me i didn't get their calls. If i'm using my phone and somebody gonna call me yea i will receive his call but when my device in sleep mode. i didn't get any calls. I've tried other sim cards. I've unlocked bootloader and flashed again stock rom but issue is still persist. I'm assuming, it should be hardware problem. It's been two months passed since I've not find any solution.
Does anyone has similar issue?
Is it software related or hardware?
I will appreciate it if anyone could help.
A man's skill is defined by his actions, not the markings on his robe.