Create a view showing total minutes per customer per month and change from previous month
Seems like you're right on the tip of the answer. Just group by the customer, the year, and then the month. 

USE Masscomm_XT; 

WITH C0 AS (
  SELECT CustomerName ,Minutes. ,year(Postingdate) AS PostingYear ,month(postingdate) AS PostingMonth ,PostingDate
  FROM dbo.reptview_LDUsageTrend
)

SELECT C0.CustomerName ,C0.PostingMonth ,C0.PostingYear, SUM (Minutes) AS MinuteTotal
FROM C0
GROUP BY C0.CustomerName, C0.PostingYear, C0.PostingMonth
;