added topics for phone-number exercises

[viveksyngh]

#856

[ilya-khadykin]

Thanks, @viveksyngh

Please also note that there are some special 'closing issue' keywords that you can put in description of your PR and Github will take care of closing the corresponding issue automatically after merge.
It makes our work much easier. I hope this hint will help you to make your future PRs a bit better

Just for future reference a guide of how to write good commit messages - https://chris.beams.io/posts/git-commit/#seven-rules

[viveksyngh]

Thanks .. I will keep above points in mind for future PRs. :)