VoiceOver - issue with ending phone calls

Hi, a blind iPhone SE user who I am helping to become proficient with VoiceOver is experiencing problems ending outgoing phone calls which I cannot replicate on my iPhone 8. She finds that her iPhone screen sometimes dims during phone calls and that, following this, double tapping with two fingers has no effect, which means that she might leave one of those interminable voice mails. On my iPhone 8 double tapping with two fingers ends calls irrespective of the state of the screen and I've not noticed problems in this area before on earlier phone models. Do others have problems ending outgoing calls? If so, what workarounds have you discovered, please?We are both running iOS 11.1.1.
Thanks, Brian