Answer
1365 samples
Work Step by Step
A sample implies that the order of selecting its contents in unimportant.
We work with combinations.
The sample with no defective phones has been chosen in a way that:
(1) 4 phones were chosen from the 15 working phones
(2) 0 phones were chosen from the 5 defective phones
Total: ${}_{15}C_{4}\displaystyle \cdot {}_{5}C_{0}=\frac{15!}{(15-4)!4!}\times\frac{5!}{(5-0)!0!}$
... by definition, $0!=1$...
$=\displaystyle \frac{15\times 14\times 13\times 12}{1\times 2\times 3\times 4}\cdot 1$
$=1365$
