WhatsApp Get Disconnected When iPhone Locked and Calls Didn't Get Through

WhatsApp sometimes get disconnected from my phone when it's locked so message gets delayed and some of the incoming calls didn't get through so i have some missed calls. I've tried contacting WhatsApp but the troubleshooting that they recommend still doesn't resolve this issue. These are the things that I have tried :

• Allowing notification for WhatsApp and have reset it's notification setting
• Resetting my iPhone Network
• Do Not Disturb mode is Off
• Making sure WhatsApp is updated
• My iOS already updated to the latest version (15.3.1)
• Background Refresh App is On for WhatsApp in Wi-fi and Cellular mode

Please kindly let me know if you guys know how to resolve this issue because it's really annoying to keep missing important calls because it didn't get through.

Thank you.