Duke Ellington wrote the music in 1939. Billy Strayhorn the lyrics.
There are several great recordings of it, one of my favourites is with Rosemary Clooney and the Michael Feinstein Big Band (1991).
This arrangement starts with an optional small telephone conversation between singer and trombone (not on audio-example). There is a swingin tutti in the middle where the horns build up their music and 2nd trombone player gives a soloing answer. Also an 8 bar solo for the 1st alto. Both soli are both written out and chordsymbols.
The key is B flat (concert) but after the tutti it modulates to C.
This is one of the recordings of the 50 years Anniversary Concert of students through the years of the Havo/vwo voor Muziek en Dans, the preparatory course of Codarts, the Rotterdam conservatory. Vocalist is Kim Hoorweg.