Call drops after a while?

my fiancée and I are usually in a call through-out the whole day, even when we sleep, but the call drops on her side after a couple of hours every time, after she already fell asleep. there are still sounds on her end, like music sometimes, or just her sleepy sounds - so I don't understand why it would drop. is there a way to prevent this from happening?

I'm on the PC app, and she's on mobile when it happens.