An ordinary call is the thing that does not work, because the ringer setting applies to calls. What works is anything that reaches the phone as a notification or a command rather than as a call.
On iPhone: Find My. Open Find My on another Apple device or at icloud.com, go to Devices, pick the phone and choose Play Sound. It plays even when the phone is muted. It needs Find My iPhone to have been turned on beforehand, under Settings > (your name) > Find My.
On Android: Find Hub, which Google used to call Find My Device. Sign in at the Find Hub site or use the app on another device, pick the phone and choose Play sound. Google documents it as ringing at full volume for five minutes even when the phone is on silent or vibrate.
A third-party app, of which RingIt is one. The difference is who triggers it: Find My and Find Hub are things you do from another device you own and are signed in to, whereas an app like RingIt lets a person you trust do it from their own phone without touching your account.
All three share the same limit. The phone has to be on and reachable. If the battery is dead or there is no connection at all, nothing gets through, and there is no app that fixes that.
How RingIt helps
RingIt covers the case where the phone is somewhere in the house and the person who can help is not you. You add them by email, they accept the invite, and from then on one tap in their app rings your phone through silent, Do Not Disturb and sleep mode: a critical alert on iOS, a bypassing alert channel on Android. Ringing is free. It makes noise and nothing else, so for a phone that has left the building, Find My or Find Hub is still the tool you want.
More on how the app fits together is in the introduction to RingIt.
