I saw this video from Refuge that gives clear instructions on how to turn the phone alert off on android and apple phones.
Obviously some people have secret phones for less than innocent reasons, however some victims of abuse may have a secret phone for emergencies or planning to leave and the alert could jeopardize their safety if this is the case. The alert will sound even if phone is on silent.
Apologies if already shared. I know it's been mentioned on other threads but couldn't see a link to the video, will be sharing generically on social media in case it helps anyone on my friend list etc.