Under WA criminal code, can I be charged criminally for having phone sex with my 16 year old boyfriend, I am over 18 years old

Of course you have all the rights guaranteed you by your state and federal constitution. The problem is that you are 18 and having some kind of sexual relation with a minor, even if it's just over the telephone. This may violate laws in WA; it would in CA. You need to consult a WA criminal lawyer.

In Washington, there is a crime entitled, "Communicating with a minor for immoral purposes" under RCW 9.68A.090. It is punishable by up to a year in jail and a $5000 fine. In practical terms, I think it's highly unlikely that any prosecutor would charge you (particularly since the age of consent for actual sex is 16), but I've seen some crazy charging decisions by prosecutors. Be careful.