SQL> create table t_test1(id number,phone varchar2(20) default null);

Table created.
SQL> CREATE SEQUENCE seq_test start with 1 increment by 1;
SQL> select seq_test.nextval from dual;

NEXTVAL

     1

SQL> insert into t_test1(phone) values ('1666');

1 row created.

SQL> select * from t_test1
2 ;

    ID PHONE
     2 1666

SQL> alter table t_test1 add a2 varchar2(20) default null;

Table altered.