if it really had no beat you wouldn't be able to map it anyway, so you'd just start the red line when the music does.
but considering its a phone ringing there is probably a regular beat to follow. If its the iphone ringtone like in this trap remix:
https://www.youtube.com/watch?v=rNn4ors94FQ (idk if its the one you're using), its an electronic marimba song and is following a beat.
the "beat" of a song isn't always played, its more inferred by the instruments' rhythm. often the percussion will strictly keep the beat, but you don't need percussion to have a beat. try just tapping "T" in the timing section to the phone ringing the same way you'd tap your foot or bob your head in time to any other song.