The paragraphs in the question follow a description of a heatwave:
The next day was broiling, almost the last, certainly the warmest, of the summer. As my train emerged from the tunnel into sunlight, only the hot whistles of the National Biscuit Company broke the simmering hush at noon. The straw seats of the car hovered on the edge of combustion [...]
The description of the weather is followed by descriptions of its effect on the bodies of the passengers in the train. The combination of heat and perspiring bodies suggests sex to the narrator:
“Hot!” said the conductor to familiar faces. “Some weather! hot! hot! hot! Is it hot enough for you? Is it hot? Is it.. .?”
My commutation ticket came back to me with a dark stain from his hand. That any one should care in this heat whose flushed lips he kissed, whose head made damp the pajama pocket over his heart!
This paragraph has been discussed previously here on Literature; it is the narrator who cares whose flushed lips the conductor kissed, though he might try to deny or minimize his feelings. Whether Carraway is momentarily attracted to the conductor, or whether the conductor’s sweating hand has triggered a memory for him, is unclear. But either way, Carraway is thinking about sex.
Then there is an ellipsis and the narration jumps to the Buchanans’ house.
Through the hall of the Buchanans’ house blew a faint wind, carrying the sound of the telephone bell out to Gatsby and me as we waited at the door.
Standing at the door, Carraway can hear the butler talking on the telephone, but the words, carried on the faint wind, are hard to understand. “Faint” is a transferred epithet: the sound, as well as the wind, is faint. What the butler really said was:
but what Carraway imagines he hears is:
“The master’s body! I’m sorry, madame, but we can’t furnish it—it’s far too hot to touch this noon!”
In this daydream Carraway imagines that a woman is on the phone (perhaps he is thinking of Myrtle Wilson in particular) and in a humorous transgression of social mores she has requested access to “the master’s” (that is, Tom Buchanan’s) body. What kind of body is this and why might Carraway daydream about it? There is a description of Buchanan in Chapter 1:
He had changed since his New Haven years. Now he was a sturdy straw-haired man of thirty with a rather hard mouth and a supercilious manner. Two shining arrogant eyes had established dominance over his face and gave him the appearance of always leaning aggressively forward. Not even the effeminate swank of his riding clothes could hide the enormous power of that body — he seemed to fill those glistening boots until he strained the top lacing, and you could see a great pack of muscle shifting when his shoulder moved under his thin coat. It was a body capable of enormous leverage — a cruel body.
Carraway does not much like Buchanan, but on the evidence of this sexually charged description he certainly appreciates Buchanan’s body.
So this brief daydream is hinting: at Buchanan’s extra-marital affair; at Carraway’s knowledge of the affair; at Carraway’s feelings of attraction to masculine bodies; and at a general sense of transgression of social boundaries.