Samsung
Set a Custom Ringtone auf Samsung
Samsung phones ship with a limited set of preloaded tones, but the Ringtone menu also reads any audio file stored on the device, including the memory card. If your own track does not appear, it usually means the file has not been indexed yet or it is saved in a format the picker does not list as a ringtone (some rare or corrupted audio files get filtered out automatically).
On dual SIM devices, Samsung lets you set a separate ringtone per SIM card, so the menu asks which line you want to change before showing the tone list.
To assign your own file:
Tap on Settings ➔ Tap on Sounds and vibration ➔ Tap on Ringtone ➔ Choose a SIM card (Dual SIM) ➔ Tap on Add ➔ Set a Custom ringtone ➔ Tap on Done
If the file you want is missing from the Add screen, move it into the main Music or Ringtones folder and reopen the picker, since a fresh scan usually solves that within seconds.