This is partly inspired by the recent question on words with both types of kana.
I am referring to words such as:
39=さんきゅう=Thank you
4649-> よろしく、46 in internet slang
5963-> ご苦労さん
I know these are often used in telephone numbers (so for example a florist would try to get a telephone number containing 8787, 花花)
It is part of modern vocabulary not something covered in text books. I found the following links but I have not found a frequency list.
http://en.wikipedia.org/wiki/Japanese_wordplay
http://www.tofugu.com/2011/08/30/goroawase-japanese-numbers-wordplay/#disqus_thread