iOS has a number of standard tones you can choose from.
But start by turning up your iPhone's ringtone volume all the way here: Settings → Sounds & Haptics
Deactivate "Change with Buttons" at the same time. This prevents the Volume Buttons from turning down the ringtone.
After that, find the menu item "Ringtones". scroll down and find the list of the Original Ringtones - try to hear the one called "Alert" or "Old Phone"
Alternatively, you can make a ringtone yourself, out of an audio file. See a guide here:
Create Ringtones in Garageband - Apple Community