From the Wikipedia entry:
The theme song, written by the show's co-writer David Croft and composer Ronnie Hazlehurst, consists of a female lift operator (whose voice was provided by Stephanie Gathercole) announcing each floor over the sounds of a cash register (which basically serves as the only percussion instrument) and a simple musical accompaniment.
There is a homage to the theme song in the Ladytron song "Paco!", and New Zealand band Minuit's "I Hate Guns." A version of the theme song is featured on the album The Ape of Naples by the experimental music group Coil. The theme song has also been covered by Australian band Regurgitator on their 1999 album ...art. Pop singer Jamelia's song "Window Shopping" (from her 2006 album Walk with Me) begins with a sample of the familiar cash register sound effect as well as Mrs. Slocombe's voice inquiring, "Are you free?". The Pink Floyd song "Money" also uses a similar cash register effect.