Oracle null constraint

I have an email and cellPhone attrib开发者_StackOverflow社区utes in a table, one of them can be null and the other must not be null, how can I make constraint to prevent two values to be null?

Something like:

CREATE TABLE mytable(   
   email varchar2(100), 
   cellphone varchar2(100),
   constraint null_check
   check (email is not null or cellphone is not null)
)