The inner join includes "only" the T1 records that match customer numbers in T2.

I want all the customer numbers in both tables to be in the resultant table

I have customer numbers that only exist in each table so they are excluded from the join?

So if there is a customer that we did not have any sales with in one of the two years, it is omitted from the result. I need them to be included in the comparison showing the sales total as zero in the year nothing exists.