in either internet shorthand or text, fwn can be a shortened version of the word "phone," by taking the "ph" sound of phone and subbing it with a "f," and then taking "own" sound from the term "pwn," and combining it with the "f"
Phone = fone
own = pwn = -wn
f-one + wn = fwn
fwn and phone are pronouced the same way.