But you have not listed them in order , you are taking the -0 as the first but 542 comes before 543 so surely the order is ....
534 542-6
534 542-7
534 542-8 
Mike Lofthouse
Maybe the batch of numbers previous to 534 542-5 have been used or allocated?
Just been looking at my motown 7's box set and noticed that they run in order of sorts but it jumps
534 543-0
534 543-1
534 543-2
then it leaps to
534 542-6
534 542-7
534 542-8
534 542-9
Are there some others with these prefix numbers