The first thing we must do for this case is to define a variable.
We have then:
x: number of phones that were sold at the wrong price.
y: number of phones that were sold with the correct price.
We now write the system of equations that represents the problem.
We have then:

Resolving the system graphically we have:

Note: see attached image for the graphic solution.
Answer:
234 phones were sold at the wrong price