You need to drop the constraint before dropping the column.

alter table drop constraint DF_Ray_Class
GO

Alter table Ray
drop column
Errno
,Errstr
,Telno
,NewTelNo
,NewTelType
,Class
Go

Alter Table Ray Add
Errno int NULL
,Errstr varchar (100)
,Telno varchar (30)
,NewTelNo varchar (20)
,NewTelType varchar (20)
,Class char (3)
CONSTRAINT [DF_Ray_Class] DEFAULT ('A')
Go