The ratio of N to D was 8:5 at first. After N decreases by 120 and D decreases by 135, the new ratio of N to D is 8:3.

Let the number of N at first be 8 old units and D at first be 5 old units. Let the number of N at the end be 8 new units and D at the end be 3 new units.

120 N phones were sold. 135 D phones were sold.

So,

8 old units - 120 = 8 new units
5 old units - 135 = 3 new units

Because 8 old units loses 120 phones to become 8 new units, every 1 old unit loses 15 phones to become 1 new unit.

So, 1 old unit - 15 = 1 new unit. This means that 3 old units - 45 = 3 new units.

Now, based on the D phones,
5 old units - 135 = 3 new units

But we now know that 3 new units = 3 old units - 45.

So,
5 old units - 135 = 3 old units - 45
2 old units = 90
1 old unit = 45

8 old units = 360

So the store imported 360 N phones at first.