You do not need to pay for this. The two options that come with the phone cost nothing, and the third one costs nothing for the part that makes noise.
Find My, on iPhone. Free with the phone. Turn it on at Settings > (your name) > Find My > Find My iPhone. When the phone is lost, open Find My on another Apple device or go to icloud.com, pick the phone under Devices and choose Play Sound, which plays with the phone muted.
Find Hub, on Android, formerly Find My Device. Free, and on most phones already on. Sign in on another device or on the web, 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.
Ringing with RingIt is free as well. What is paid is the customisation: the ring sound, the three volume levels, how long it rings and the widgets. The ring itself is not behind a paywall.
The thing that actually costs you is time, and you have to spend it in advance. Every option here needs an account signed in, a setting turned on or an app installed while the phone is still with you. There is no free trick that works on a phone you have already lost and never prepared.
How RingIt helps
RingIt covers a gap the free built-in tools leave: they assume you have another device of your own, signed in and to hand. RingIt lets a person you invited by email ring your phone from theirs, in one tap, through silent, Do Not Disturb and sleep mode. That does not replace Find My or Find Hub, which show you where the phone is and can erase it if it is stolen. RingIt only makes noise, and only while the phone is on and online.
More on how it works is in the introduction to RingIt.
