"He has a black cell phone."
Translation:Hänellä on musta kännykkä.
I don't know of any "-ellä" suffix, but if you're referring to the -lla/-llä suffix, that's triggered by adessive case. An adessive case ending should be used:
- to express the static state of being on the surface of something.
- as an existential clause with the verb "olla" (to be) to express possession.
- to express the instrumentive use of something
- in certain time expressions expressing the time at which things take place
- to express the general proximity in space or time at which something takes place (the more specific proximity case would be the inessive)
- in certain expressions expressing mood
The -lla suffix is used after back vowels and the -llä suffix is used after front vowels.
I should add that the "e" between "hän" and the adessive ending is a genitive stem. Adessive singular is formed by adding a genitive stem then adding an adessive ending. The formation of a genitive stem is unfortunately governed by a rather complicated set of rules, which are these: http://users.jyu.fi/~pamakine/kieli/suomi/sijat/genetiivivaren.html
Forming a plural adessive requires a plural stem instead of a genitive stem. Formation of a plural stem works like this: http://users.jyu.fi/~pamakine/kieli/suomi/sijat/monikkovartaloen.html