SQL Server 2017

By using db<>fiddle, you agree to license everything you submit by Creative Commons CC0.

CREATE TABLE Numbers (Number INT NOT NULL PRIMARY KEY);
WITH e1(n) AS (SELECT 1 FROM (VALUES (1),(1),(1),(1),(1),(1),(1),(1),(1),(1)) n (n)), -- 10
e2(n) AS (SELECT 1 FROM e1 CROSS JOIN e1 AS b), -- 10*10
e3(n) AS (SELECT 1 FROM e1 CROSS JOIN e2) -- 10*100
INSERT dbo.Numbers (Number)
  SELECT n = ROW_NUMBER() OVER (ORDER BY n) FROM e3 ORDER BY n;
1000 rows affected

 CREATE TABLE [PHONE_CALLS] 
 (
	Call_ID INT IDENTITY(1, 1) NOT NULL, 
	ArrivalTime DATETIME NOT NULL
);
 INSERT [PHONE_CALLS] (ArrivalTime)
 SELECT DATEADD(SECOND, -500000 * RAND(CHECKSUM(NEWID())), GETDATE())
 FROM dbo.Numbers;
1000 rows affected

DECLARE @TrendStartDate DATE = '2020-01-06',
	@TrendEndDate DATE = '2020-01-08';
		
SELECT  d.Date,
        h.Hour,
        Calls = COUNT(pc.Call_ID)
FROM    (   SELECT  [Hour] = Number
            FROM    dbo.Numbers 
            WHERE   Number >= 0
            AND     Number < 24
        ) AS h
        CROSS JOIN
        (   SELECT  Date = DATEADD(DAY, Number, @TrendStartDate)
            FROM    dbo.Numbers 
            WHERE   Number <= DATEDIFF(DAY, @TrendStartDate, @TrendEndDate)
        ) AS d
        LEFT JOIN [PHONE_CALLS] AS pc
            ON CAST(pc.ArrivalTime AS DATE) = d.Date
            AND DATEPART(HOUR, pc.ArrivalTime) = h.Hour
GROUP BY d.Date, h.Hour
ORDER BY d.Date, h.Hour;            


Date
Hour
Calls


2020-01-07
1
0

2020-01-07
2
0

2020-01-07
3
0

2020-01-07
4
0

2020-01-07
5
0

2020-01-07
6
0

2020-01-07
7
0

2020-01-07
8
0

2020-01-07
9
0

2020-01-07
10
0

2020-01-07
11
0

2020-01-07
12
0

2020-01-07
13
0

2020-01-07
14
0

2020-01-07
15
0

2020-01-07
16
0

2020-01-07
17
0

2020-01-07
18
0

2020-01-07
19
0

2020-01-07
20
0

2020-01-07
21
0

2020-01-07
22
0

2020-01-07
23
0

2020-01-08
1
0

2020-01-08
2
0

2020-01-08
3
0

2020-01-08
4
0

2020-01-08
5
0

2020-01-08
6
0

2020-01-08
7
0

2020-01-08
8
0

2020-01-08
9
0

2020-01-08
10
0

2020-01-08
11
0

2020-01-08
12
0

2020-01-08
13
0

2020-01-08
14
0

2020-01-08
15
0

2020-01-08
16
0

2020-01-08
17
0

2020-01-08
18
0

2020-01-08
19
0

2020-01-08
20
0

2020-01-08
21
0

2020-01-08
22
0

2020-01-08
23
0


Warning: Null value is eliminated by an aggregate or other SET operation.

Date	Hour	Calls
2020-01-07	1	0
2020-01-07	2	0
2020-01-07	3	0
2020-01-07	4	0
2020-01-07	5	0
2020-01-07	6	0
2020-01-07	7	0
2020-01-07	8	0
2020-01-07	9	0
2020-01-07	10	0
2020-01-07	11	0
2020-01-07	12	0
2020-01-07	13	0
2020-01-07	14	0
2020-01-07	15	0
2020-01-07	16	0
2020-01-07	17	0
2020-01-07	18	0
2020-01-07	19	0
2020-01-07	20	0
2020-01-07	21	0
2020-01-07	22	0
2020-01-07	23	0
2020-01-08	1	0
2020-01-08	2	0
2020-01-08	3	0
2020-01-08	4	0
2020-01-08	5	0
2020-01-08	6	0
2020-01-08	7	0
2020-01-08	8	0
2020-01-08	9	0
2020-01-08	10	0
2020-01-08	11	0
2020-01-08	12	0
2020-01-08	13	0
2020-01-08	14	0
2020-01-08	15	0
2020-01-08	16	0
2020-01-08	17	0
2020-01-08	18	0
2020-01-08	19	0
2020-01-08	20	0
2020-01-08	21	0
2020-01-08	22	0
2020-01-08	23	0