After you obtain Denise as you girlfriend after the Burning Desire mission, listen to Radio Los Santos. At times between songs, Julio G. (the DJ) will have a shout-out from Denise to you, and at the end he criticizes the shout out by saying "I got a shout out from Denise in Ganton who says she misses her boy desperately. And I quote, desperately. Hey, you know, this isn't Lonely Hearts, this is Radio Los Santos."