Hikvision Doorbell 6003 - Not ringing phone when doorbell is pressed

I have just purchased this doorbell and I'm having a problem with this doorbell not calling our phones (Samsung S8 on the latest OS) when the doorbell button is pressed. It rings occasionally, this tends to be when the phone is in use or has just been used or the app is open. If the phone goes into rest mode the doorbell ring notification will not work. I get the missed call notification, after the event and the motion detection notification comes through okay it just wont ring the phone at...

Hikvision Doorbell 6003 - Not ringing phone when doorbell is pressed