Apple iPhone 12 / No Ringtone

[BizWebConsulting]

I've received a call from a client just now who is using the 3CX Startup and an Apple iPhone 12.
The customer is advising that there's no volume when there's a call and he's generally getting a "missed call" notification.
It was working when we had set it up originally but all of a sudden, there's no sound.
We did experiment just now to see if we could replicate it and trying to increase "sound volume" on the ringer but to no avail.
I've seen some of the other threads but there's not these options with the StartUP system.

 

[Vali_3CX]

1 - 3CX client version?
2 - iOS version?
3 - If the device is locked, 3CX client is not ringing or is ringing but there is no ringtone?