Deletes her number...
Misses her...
Downloads that song...
And the bass-guitar at the beginning invites to memory, the scent of her conditioned hair.
When the lyrics reach the second line and verse?
Her number comes back to mind- jus' like that.  
So he writes it down, saves it back to his phone.
And now...
He remembers she said; "You'll think of me"
-before the door slammed...