Validate mobile number in a database making sure it does not exist in other database

Create a Stored Procedure with 2 parameters (input PhoneNumber, output Exists) that contains a UNION query for all 4 tables. Then in C# you can call that Stored Procedure with those 2 parameters and find out if it exists or not.

SQL Stored Procedure

CREATE PROCEDURE dbo.usp_PhoneNumberCheck (
	@PhoneNumber	NVARCHAR(100),
	@Exists		BIT	OUTPUT	= 1
) 
AS
BEGIN
	; WITH cte AS (
		SELECT PhoneNumber FROM DatabaseName1.SchemaName.TableName
		UNION
		SELECT PhoneNumber FROM DatabaseName2.SchemaName.TableName
		UNION
		SELECT PhoneNumber FROM DatabaseName3.SchemaName.TableName
		UNION
		SELECT PhoneNumber FROM DatabaseName4.SchemaName.TableName
	)

	IF NOT EXISTS ( SELECT 1 FROM cte WHERE PhoneNumber = @PhoneNumber) BEGIN
		SET @Exists = 0
	END
END
GO

C# method guts

string PhoneNumber = "9115551212"; // placeholder
bool NumberExists = true;

using (SqlConnection conn = new SqlConnection("ConnectionString")) {
	SqlCommand cmd = new SqlCommand("dbo.usp_PhoneNumberCheck", conn);
	cmd.CommandType = CommandType.StoredProcedure;

	SqlParameter outputExists = new SqlParameter("@Exists", SqlDbType.Bit) {Direction = ParameterDirection.Output};

	cmd.Parameters.AddWithValue("@PhoneNumber", PhoneNumber);
	cmd.Parameters.Add(outputExists);

	conn.Open();
	cmd.ExecuteNonQuery();

	NumberExists = outputExists.value;

	conn.Close();
}