let a = 1st digit, b 2nd digit , c 3rd digit

100a + 10b + c

Multiply by 80 ...

8000a + 800b + 80c

Add 1 ...

8000a + 800b + 80c + 1

Multiply by 250 ...

2000000a + 200000b + 20000c + 250

Add the last 3 digits of your phone number with a 0 at the end as one number eg, 5670

let d = 1st digit, e 2nd digit , f 3rd digit

2000000a + 200000b + 20000c + 1000d + 100e + 10f + 250

Repeat step 6 ...

2000000a + 200000b + 20000c + 2000d + 200e + 20f + 250

Subtract 250 ...

2000000a + 200000b + 20000c + 2000d + 200e + 20f

Divide number by 20 ...

you finish it.