A depreciation rate of 8.25% means that year on year the phones are valued at 91.75% of their value.
If t is the number of years, the value v=999(0.9175)ᵗ dollars after t years. At t=0, the value is $999.
ln(0.9175)=-0.0861 so v=999e⁻⁰˙⁰⁸⁶¹ᵗ is another way of writing the function.