#### Answer

See the explanation below.

#### Work Step by Step

The joke tells us to multiply the imaginary number with $i$.
The imaginary number is the product of a real number with $i$.
For example, $5i$
The imaginary unit $i$ is the solution of the equation${{x}^{2}}=-1$ and its value is equal to$\sqrt{-1}$.
The product of imaginary number with $i$ gives a real number.
$\begin{align}
& \left( \text{Imaginary number} \right)\cdot i=\left( 5i \right)\cdot i \\
& =5{{i}^{2}} \\
& =5\left( -1 \right) \\
& =-5
\end{align}$
If the imaginary number is multiplied with $i$ then the number obtained will be a real number and hence, it is possible to dial the number.
Therefore, the multiplication of the imaginary number with $i$ gives a real number.