This is partly inspired by the recent question on words with both types of kana.
I am referring to words such as:
4649-> よろしく、46 in internet slang
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.