Need to convert date to word using trigger

I have created a datetoword function in Mysql. Checked the function working fine. Now using this function need to convert the date from field1 (date-datetype) to field2 (Varchar-datatype) within the table using trigger on before insert.

Tried with some trigger code

Set New.dobw = CALL date_to_words(New.dob);

Help needed in this regard.