How to write the query with not equals to ?

Hi ,

Table 1:

CREATE TABLE [dbo].[helpdesk](
[CallOwnerId] [varchar](255)  NULL,
[CallOwner] [varchar](255)  NULL,
[CallOwnerTelephone] [varchar](50)  NULL )

Table 2:

CREATE TABLE [dbo].[dim_helpdesk](
[call_owner_code] [varchar](255)  NULL,
[call_owner_telephone] [varchar](50)  NULL )

insert into [dbo].[dim_helpdesk](
[call_owner_code] ,
[call_owner_telephone] )values ('janet_owen','_27 11 239 6210')

insert into [dbo].[helpdesk](
[CallOwnerId] ,
[CallOwner],CallOwnerTelephone) values ('janet_owen','Janet Owen',NULL)

My Requirement : how to select where the ph no doesnot match like the above record.