The comma is wrong.
Commas in this context are used to introduce clauses which add information but don't change the meaning.
I called John, who is on vacation.
is OK because it tells us exactly who I called (John) and added the additional information that he is on vacation. If you omitted the clause after the comma it would not change the meaning of the first part.
In your example case if you omit the clause after the comma then you don't know which phase it is talking about: "The first step is the phase" doesn't make sense. The part after the comma is essential to the meaning and so shouldn't have a comma.