What is it called when single letters are used as words in a sentence?

What is it called when single letters are used as words in a sentence?

I C A B.
I C D B P!

I see a Bee.
I see the bee pee!

F U N-E X?
S I F X.
F U N-E M?
S I F M.
OK I L F M N X!

Have you any eggs?
Yes I have eggs.
Have you any Ham?
Yes I have ham.
OK. I will have ham and eggs.

I have been looking for this, at a low level, for years.