We need to decide on 7 digits so we will use 7 stars. The bars will represent a switch from each possible single digit number down to the next smaller one. So the phone number 866-5221 is represented by the stars and bars chart

\begin{equation*} |*||**|*|||**|*|\text{.} \end{equation*}

There are 10 choices for each digit (0-9) so we must switch between choices 9 times. We have 7 stars and 9 bars, so the total number of phone numbers is

\begin{equation*} {16 \choose 9}\text{.} \end{equation*}