How to set off someone else's phone without find my iPhone?

My mum lost her phone today and we've looked everywhere for it. We tried ringing it but it's only silent it's definitely not flat. We tried doing findd my iPhone but it turns out she didn't know how to set it up when she got her phone so we can't play the sound it's definitely around the house somewhere so I was just wondering if there's any other away to trigger the find my phone sound

Without FindMyiPhone activated? You don't. Just think about how much pranking there would be if that was possible…

No there isn't. That's precisely why your mum was prompted to set it up before she lose it.

She's now going to have to look for it the old-fashioned way by methodically searching every part of every room until she finds it. Obviously if the phone is at home and you've both really looked everywhere for it you'd have found it, so your mum is now really going to have to look everywhere. On the plus side the house should be a lot tidier afterwards…

You don't. Happy searching.