Sql – Violation of UNIQUE KEY constraint during SQL update

I have a simple database table (SQL Server 2008 R2 Express), which has a definition as follows:

ID         INT          Auto Inc, PK
Name       VARCHAR(64)  Unique Key
Telephone  VARCHAR(128)

I have a stored procedure which I execute to update records within the table which basically does the following:

UPDATE customers
SET    Name = @name, Telephone = @Telephone
WHERE  id = @id

Currently, I have two entries in the table

ID   Name    Telephone
1    Fred    01234 567890
2    John    09876 543210

When I call my stored procedure to update the telephone number for John, the SQL that is effectively executed is

UPDATE customers
SET    Name = 'John', Telephone = '02468 135790'
WHERE  id = 2

This generates a UNIQUE KEY violation on the Name field. Now as the Name field doesn't actually change, why does this occur?

As all database actions are being handled by my app using stored procedures, I could fix this by removing the constraint, and modifying the stored procedures to manually enforce the constraint, but this just seems wrong.

Given that my table actually has many more fields, there must be a generic work around that I can employ to prevent these false constraint problems, without having to generate numerous stored procedures to update specific fields?

Edit: The above table was simplified to keep the question more manageable, I'm pretty sure I've not missed anything important, but for info, the actual definition of the table is as follows

SET ANSI_NULLS ON
GO

SET QUOTED_IDENTIFIER ON
GO

SET ANSI_PADDING ON
GO

CREATE TABLE [dbo].[companies](
    [id] [int] IDENTITY(1,1) NOT NULL,
    [typeId] [int] NOT NULL,
    [name] [varchar](64) NOT NULL,
    [displayName] [varchar](128) NOT NULL,
    [deliveryAddress] [varchar](1024) NOT NULL,
    [invoiceAddress] [varchar](1024) NOT NULL,
    [telephone] [varchar](64) NOT NULL,
    [fax] [varchar](64) NOT NULL,
    [email] [varchar](256) NOT NULL,
    [website] [varchar](256) NULL,
    [isActive] [bit] NOT NULL,
 CONSTRAINT [PK_companies] PRIMARY KEY CLUSTERED 
(
    [id] ASC
)WITH (PAD_INDEX  = OFF, STATISTICS_NORECOMPUTE  = OFF, IGNORE_DUP_KEY = OFF, ALLOW_ROW_LOCKS  = ON, ALLOW_PAGE_LOCKS  = ON) ON [PRIMARY],
 CONSTRAINT [Unique Display Name] UNIQUE NONCLUSTERED 
(
    [displayName] ASC
)WITH (PAD_INDEX  = OFF, STATISTICS_NORECOMPUTE  = OFF, IGNORE_DUP_KEY = OFF, ALLOW_ROW_LOCKS  = ON, ALLOW_PAGE_LOCKS  = ON) ON [PRIMARY],
 CONSTRAINT [Unique Name] UNIQUE NONCLUSTERED 
(
    [name] ASC
)WITH (PAD_INDEX  = OFF, STATISTICS_NORECOMPUTE  = OFF, IGNORE_DUP_KEY = OFF, ALLOW_ROW_LOCKS  = ON, ALLOW_PAGE_LOCKS  = ON) ON [PRIMARY]
) ON [PRIMARY]

GO

SET ANSI_PADDING OFF
GO

ALTER TABLE [dbo].[companies]  WITH CHECK ADD  CONSTRAINT [Company Type] FOREIGN KEY([id])
REFERENCES [dbo].[companyTypes] ([id])
GO

ALTER TABLE [dbo].[companies] CHECK CONSTRAINT [Company Type]
GO

…and the stored procedure

ALTER PROCEDURE UpdateCompany

    @id                 INT,
    @typeId             INT,
    @name               VARCHAR(64),
    @displayName        VARCHAR(128),
    @deliveryAddress    VARCHAR(1024),
    @invoiceAddress     VARCHAR(1024),
    @telephone          VARCHAR(64),
    @fax                VARCHAR(64),
    @email              VARCHAR(256),
    @website            VARCHAR(256),
    @isActive           BIT

AS
BEGIN

    UPDATE  companies
    SET     typeid = @typeid,
            name = @name,
            displayname = @displayname,
            deliveryAddress = @deliveryAddress,
            invoiceAddress = @invoiceAddress,
            telephone = @telephone,
            fax = @fax,
            email = @email,
            website = @website,
            isActive = @isActive

    EXEC    GetCompany @id


END
GO