The most important thing to gather from this prologue is the complex framing devices Plato establishes before telling his story. We might contrast the Phaedo with The Apology, where a direct transcript of Socrates' trial is given, and we are explicitly told that Plato was present at the trial. Here, on the other hand, we receive a second-hand account--Phaedo is telling the story to Echecrates long after the fact--and Plato's absence at Socrates' death is explicitly mentioned. A clear effort is being made to distance Plato, the author, from the story being narrated. One possible explanation could be that the Phaedo is a dialogue of Plato's middle period, and contains many of Plato's own doctrines. Plato does not want these doctrines to be associated with the historical Socrates, and so goes to some lengths to distance himself from the events narrated as if to suggest that he is by no means an authority on the factual events of Socrates' death. Instead, this is a fictitious account of Plato's idealized version of Socrates' last hours. This may also be a directive to read the dialogue philosophically: The author has distanced himself sufficiently from the events that we cannot be certain as to whether or not the tale itself is true; as a result, we will not be distracted from the actual philosophical arguments by questions of whether or not the account is plausible.
Simmias and Cebes are the primary interlocutors of the Phaedo, and neither are Athenians. Simmias is from Thebes, and Cebes from Phaedondas. Both would have been happy to welcome Socrates into their native towns had he chosen to escape prison (see the Crito). Both are also Pythagorean philosophers, as is Phaedo, a significant point in a dialogue that borrows so heavily from Pre-Socratic philosophy.
The Pythagoreans are followers of Pythagoras, the inventor of mathematics. Among their beliefs are the worship of numbers, the belief that proper combinations of numbers applied in music can create beautiful harmonies they call "the music of the spheres," and a belief in reincarnation--that after death, the soul migrates into another body and is reborn. It is important that the interlocutors for this dialogue are fellow philosophers rather than common people. The Phaedo is one of Plato's more philosophically substantial dialogues, and his choice of interlocutors suggests that he is addressing this work to fellow philosophers rather than to a general audience.
The introduction here of the relation between pleasure and pain is a prelude to further discussion. A main theme of the dialogue is the distinction between bodily sensations and the life of the mind, or soul. While apparent opposites, pleasure and pain are both physical sensations, and therefore are both to be equally despised by the serious philosopher.