Set Transaction Use Rollback Segment

[kgb]

Hi all
Could you pleas have a look the code bellow and tell me please what I am doing wrong? Looks like transaction goes to wrong rollback segment.
Thanks

SQL> SET TRANSACTION USE ROLLBACK SEGMENT Rollbig;

Transaction set.

SQL> alter table calls drop unused columns;
alter table calls drop unused columns
*
ERROR at line 1:
ORA-01562: failed to extend rollback segment number 1
ORA-01650: unable to extend rollback segment R01 by 4 in tablespace RBS

Oracle 8.1.7.4, Linux RedHat 6.2