Cute. I was obsessively compelled to break it apart. So...

When you multiply by 80 and then by 250, it's the same as multiplying by

(2 x 2 x 2 x 2 x 5) = 80

(2 x 5 x 5 x 5) = 250

if you regroup these 2 x (2x5) x (2x5) x (2x5) x (2x5) = 80x250
its the same as 2 x 10000.

lets call the first three digits of your phone # "ABC"
2 x 10000 x (ABC) = (2) x A,BC0,000

But, we had added a 1 that also got multiplied by 250. so that 'adds' 250 to this. So the sum at this point is

(2) x A,BC0,000 + 250 = (2) x A,BC0,250

But then all you do is add and subtract for steps 5, 6 and 7 and since it doesn't matter what order you add and subtract stuff, lets do step 7 first and subtract 250.

Now you still have
(2) x A,BC0,000

Let's call your last 4 digits "DEFG"
You add them twice, which is the same as adding (2) x DEFG so we can write this out as

[(2) x A,BC0,000] + [(2) x D,EFG]

equals

(2) x (A,BC0,000 + D,EFG)
(2) x (A,BCD,EFG)

Step 8 divide by 2

and you have A,BCD,EFG

neat :-)