The sound comes from One Piece, the long running manga and anime series created by Eiichiro Oda. In that world, Den Den Mushi are living snail phones, and their calls are signalled by a repeated poro poro poro sound instead of a normal ringtone. Clips of the call spread online as ringtones, edits and soundboard buttons, especially among anime fans who recognised the reference instantly. It stays popular because the loop is distinctive, easy to recognise in a few seconds, and useful whenever a creator wants a phone call to feel unmistakably cartoon and anime styled.