"My cell phone is lost."
Translation:Cep telefonum kayboldu.
Hello, Dr.M.saeed. The bit of research I've done indicates that they are not interchangeable, though related.
Here is a brief discussion of these very words: http://www.turkishclass.com/forumTitle_60796
Also, I have not confirmed this, but I see that the word kayıp can be a noun ('loss') or an adjective ('lost,' 'missing'). I'm thinking that these two verbs may relate to kayıp in this way:
kayıp olmak → kaybolmak
kayıp etmek → kaybetmek
(Evidence for this from https://en.wikipedia.org/wiki/Turkish_grammar#Auxiliary_verbs)
Whatever the case, their basic meanings are
kaybolmak : to be lost, disappear
kaybetmek : to lose