XS sounds like excess, B4 sounds like before, XLNC sounds like excellency, etc. What are these wordplays called?
After some googling, I found the term grammagram referring to this type of wordplay. While not as common/official as rebus, this term is much more specific, since a rebus is usually mostly pictures, with few letters. It is an RL-coined (Richard Lederer) word, apparently (see this thefreelibrary entry), which, together with the comparative rarity of grammagrams themselves, would explain its uncommon status.
Personally, I'd use it, though rebus (or letter rebus) would be more widely understood.
That sort of wordplay is called a rebus.