Connection is disconnected when the phone screen is turned off

[YYH123123]

I am making an voice chat app without video. Due to your advice, Audio is working well now.

But the problem is that when the phone screen is turned off(because of touching power button or because the screen is automatically turned off as specific time passes), the connection is disconnected making the voice chat end.
Can you let me know how to keep the connection even though the phone screen is turned into black?

Please help me:(