[v3] Extract Concepts from v2 exercise: phone-number

Extract the list of concepts required to solve the v2 C++ exercise: `phone-number`. For each concept, describe why it is required to solve the exercise. Keep in mind that an exercise can have multiple approaches (you can find community solutions here: `https://exercism.io/tracks/cpp/exercises/phone-number/solutions`). The goal of this task is to help determine the total list of concepts that make up `phone-number`.

The list of concepts should be stored in the following document: `languages/cpp/reference/exercise-concepts/phone-number.md`. [Use this link to create a PR to add this document][pr-link].

[pr-link]: ../new/master?filename=languages/cpp/reference/exercise-concepts/phone-number.md

Create languages/cpp/reference/exercise-concepts/phone-number.md. Start by reading the v2 phone-number exercise and reviewing the linked community solutions to identify the concepts used by different approaches. Done means the document lists each required concept and explains why it is needed for the exercise.

Escrito por el modelo de indexación a partir del texto del issue.