How do I dedupe multiple database fields

I have a source table with the schema:
ContactName varchar
Tel1 varchar
Tel2 varchar
Tel3 varchar
DateToContact datetime

This contains around 5million records.
When the DateToCall is within the next 14 days I append the data into a table with the same schema. At the same time I also remove data where the DateToCall has passed. So the table only ever contains the data that needs to be called in the next 14 days.

When the data is appended I need to check whether the data in any of the 3 telephone fields is in any of the 3 fields that is already in the table. So that no telephone number appears more than once across all 3 fields.

I am currently using a while loop to do this, looking for duplicates before each insert, but its very inefficient. Its taking around (16 hours to complete each night).

I thought of using a table to hold a list of the numbers in the destination table, populated with the data in all 3 tel fields but this goes out of date as soon as record is appended, so I am back to procesing the data one row at a time to ensure there are no duplicates.

Has anyone got any better ideas?

The code I am currently using is as follows:

SET NOCOUNT ON

DECLARE @record_id INT
DECLARE @ContactName varchar(200)
DECLARE @tel1 varchar(200)
DECLARE @tel2 varchar(200)
DECLARE @tel3 varchar(200)
DECLARE @DateToContact datetime
DECLARE @record_count1 INT
DECLARE @record_count2 INT
DECLARE @record_count3 INT

IF OBJECT_ID('TempDB..#mydata','U') IS NOT NULL BEGIN
      DROP TABLE #mydata
END

CREATE TABLE #mydata (
      id INT IDENTITY(1,1),
      ContactName varchar(200) NULL,
      tel1 varchar(200) NULL,
      tel2 varchar(200) NULL,
      tel3 varchar(200) NULL,
      DateToContact DATETIME NULL,
      CONSTRAINT [PK_mydata] PRIMARY KEY CLUSTERED ([id] ASC)      
)

INSERT #mydata
      (ContactName,
      tel1,
      tel2,
      tel3,
      DateToContact)
      
SELECT
      ContactName,
      tel1,
      tel2,
      tel3,
      DateToContact
FROM
      dbo.SourceTable
WHERE
      DATEDIFF(dd,GETDATE(),DateToContact) < 15 AND
      (tel1 IS NOT NULL OR tel2 IS NOT NULL OR tel3 IS NOT null)

DECLARE @counter INT
DECLARE @idcount int
DECLARE @name varchar(40)
DECLARE @sql varchar(max)

SET @counter = 1
SELECT @idcount = MAX(id)FROM #mydata

WHILE @counter <= @idcount BEGIN

      SELECT
            @ContactName = ContactName,
            @tel1 = tel1,
            @tel2 = tel2,
            @tel3 = tel3,
            @DateToContact = DateToContact
      FROM
            #mydata
      WHERE
            id = @counter
      
      SET @record_count1 = 0
      SET @record_count2 = 0
      SET @record_count3 = 0
      
      SELECT
            @record_count1 = COUNT(source_record_id)
      FROM
            DestinationTable
      WHERE
            (tel1 IN (@tel1, @tel2, @tel3))
            
      SELECT
            @record_count2 = COUNT(source_record_id)
      FROM
            DestinationTable
      WHERE
            (tel2 IN (@tel1, @tel2, @tel3))
            
      SELECT
            @record_count3 = COUNT(source_record_id)
      FROM
            DestinationTable
      WHERE
            (tel3 IN (@tel1, @tel2, @tel3))      
            
      IF @record_count1 + @record_count2 + @record_count3 = 0 BEGIN      
            PRINT 'Inserting no ' + @tel1
            INSERT
                  DestinationTable
                  (ContactName,
                  tel1,
                  tel2,
                  tel3,
                  DateToContact)
            VALUES
                  (@ContactName,
                  @tel1,
                  @tel2,
                  @tel3,
                  @DateToContact)
      END
      SET @counter = @counter + 1
END

DROP TABLE #mydata

SET NOCOUNT OFF