Hi raghavendar rk,
Welcome to the Apple Support Communities!
I understand that your iPhone is becoming unresponsive on the Keypad section of your Phone application when using a Bluetooth device or headphones. To begin troubleshooting, I would suggest restarting your iPhone. If the issue continues after a restart, please reset your iPhone. If the issue still continues, please let me know.
Restart or reset your iPhone, iPad, or iPod touch - Apple Support
Best regards,
Joe