Tell them to multiply that number by 80.
After that equation, tell them to add one.
Next, have them multiply that number by 250.
Tell them to add the last four digits of their number to this figure
Then do exactly the same thing, once more.
Subtract 250 from this figure.
Divide the entire thing by two.
Bask in the glory of their facial expression, because you just got their phone number via calculator!