This is actually a serious issue for me. I'm a lawyer, and I have had clients have their phones confiscated when it started ringing in court. The judge was going to hold it for a month as evidence in a contempt hearing, but I was able to get it back for the client at the end of the day when I explained he didn't understand the instructions at the beginning of the day because of his poor English skills. I would not be able to rely on that excuse.
Our courts also generally lack public wifi, so I'm very dependent on my phone as a hotspot. I can't simply leave it off. If it randomly reboots or I have to reboot because of an error, I'm in serious trouble. I have enabled the PIN at bootup for extra security, so it's even more frustrating.
It can't be that hard for the developers to just remove the bootup sound entirely. It adds nothing to the experience of using the phone and is a significant disincentive to buying another Blackview in the future.