If I understand you correctly, this might help you:
DECLARE @mockup TABLE(ID INT IDENTITY,YourExample VARCHAR(100));
INSERT INTO @mockup VALUES
('IS-1, IS-12, IS-123, IS-123.11, IS-123.a.')
,('IS-1170, IS-1171, IS-1172, IS-1173, IS-1174, IS-870.a, IS-871.a, IS-872.a');
WITH Splitted AS
(
SELECT *
,CAST('<x>' + REPLACE(m.YourExample,',','</x><x>') + '</x>' AS XML) AS SplitAtComma
FROM @mockup AS m
)
,NumberExtracted AS
(
SELECT s.ID
,part.value('text()[1]','nvarchar(max)') AS OnePart
,CAST('<y>' + REPLACE(REPLACE(part.value('text()[1]','nvarchar(max)'),'.','-'),'-','</y><y>') + '</y>' AS XML).value('/y[2]/text()[1]','int') AS TheNumber
FROM Splitted AS s
CROSS APPLY s.SplitAtComma.nodes('/x') AS A(part)
)
SELECT *
FROM NumberExtracted
ORDER BY ID,TheNumber;
The first CTE uses a string-split via XML to get all values within the original string (btw: never store comma separated values!).
The second CTE will use the same approach to extract the number, typesafe as INT.
You can use this in an ORDER BY
finally.
The result:
+----+-----------+-----------+
| ID | OnePart | TheNumber |
+----+-----------+-----------+
| 1 | IS-1 | 1 |
+----+-----------+-----------+
| 1 | IS-12 | 12 |
+----+-----------+-----------+
| 1 | IS-123 | 123 |
+----+-----------+-----------+
| 1 | IS-123.11 | 123 |
+----+-----------+-----------+
| 1 | IS-123.a. | 123 |
+----+-----------+-----------+
| 2 | IS-870.a | 870 |
+----+-----------+-----------+
| 2 | IS-871.a | 871 |
+----+-----------+-----------+
| 2 | IS-872.a | 872 |
+----+-----------+-----------+
| 2 | IS-1170 | 1170 |
+----+-----------+-----------+
| 2 | IS-1171 | 1171 |
+----+-----------+-----------+
| 2 | IS-1172 | 1172 |
+----+-----------+-----------+
| 2 | IS-1173 | 1173 |
+----+-----------+-----------+
| 2 | IS-1174 | 1174 |
+----+-----------+-----------+