"My sister spoke with her friend over cell phone."
Translation:Mi hermana habló con su amigo por celular.
I would say ¨over the phone¨ but never ¨over cell phone¨ but i would use ¨by cell phone¨which DL does not allow. In English you can always talk ¨by¨ the instrument you use (by phone), you can go ¨by¨the method of travel (by car) and die ¨by¨the cause of death (death by homicyde).
A little frustrating that suddenly we cannot use a core principle of English.