SQL Server 2014

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DECLARE @separator CHAR(1) = ',';

WITH testTable   (PhoneNumber,Activity,ActivityDate) as 
(
  SELECT 9075551234 , 'Incoming' , '2022-04-01'    UNION ALL
  SELECT 9075551234 , 'Outgoing' , '2022-04-06'    UNION ALL
  SELECT 9075551234 , 'Outgoing' , '2022-04-10'    UNION ALL
  SELECT 9075551234 , 'Outgoing' , '2022-08-02'    UNION ALL
  SELECT 9075551234 , 'Incoming' , '2022-08-05'    UNION ALL
  SELECT 9075551234 , 'Lateral' ,  '2022-08-10'     UNION ALL
  SELECT 5551239876 , 'Incoming' , '2022-07-01'    UNION ALL
  SELECT 5551239876 , 'Outgoing' , '2022-07-06'    UNION ALL
  SELECT 5551239876 , 'Outgoing' , '2022-08-01'    UNION ALL
  SELECT 5551239876 , 'Outgoing' , '2022-08-02'    UNION ALL
  SELECT 5551239876 , 'Incoming' , '2022-08-15'   
),
CTE2 as(
SELECT PhoneNumber,Activity,ActivityDate
, CASE WHEN DATEDIFF(Day,LAG(ActivityDate) OVER (PARTITION BY PhoneNumber ORDER BY  ActivityDate),ActivityDate) > 90 THEn 1 ELSE 0 END rn FROM testTable 
)
, CTE3 as (
SELECT
PhoneNumber,Activity,ActivityDate, SUM(rn)  OVER (PARTITION BY PhoneNumber ORDER BY  ActivityDate) rn2
FROM CTE2 
)
SELECT p.phonenumber
,   min(activitydate) FirstContact
,max(activitydate) LatestContact 
, STUFF((SELECT @separator + Activity 
          FROM CTE3 AS c
          WHERE c.phonenumber = p.phonenumber AND c.rn2 = p.rn2

FOR XML PATH('')), 1, LEN(@separator), '') AS Result
FROM CTE3 AS p
GROUP BY p.phonenumber,rn2  
ORDER BY p.phonenumber;

phonenumber	FirstContact	LatestContact	Result
5551239876	2022-07-01	2022-08-15	Incoming,Outgoing,Outgoing,Outgoing,Incoming
9075551234	2022-04-01	2022-04-10	Incoming,Outgoing,Outgoing
9075551234	2022-08-02	2022-08-10	Outgoing,Incoming,Lateral