-- why cast [Phone no] AS VARCHAR(MAX)?

-- why use CHARINDEX? You don't need the character position, you only need to know if

-- one string exists inside the other.

SELECT [customer].[Customer name],[customer].[Sl_No],[customer].[Id]

FROM [company].dbo.[customer]

WHERE ( Charindex('9000413237',CAST([company].dbo.[customer].[Phone no] AS VARCHAR(MAX)))>0 )

-- use this instead

SELECT c.[Customer name],c.[Sl_No],c.[Id]

FROM [company].dbo.[customer] c

WHERE c.[Phone no] LIKE '%9000413237%'

-- then try this, which is SARGable (can use a suitable index)

SELECT c.[Customer name],c.[Sl_No],c.[Id]

FROM [company].dbo.[customer] c

WHERE c.[Phone no] LIKE '9000413237%'