Why are the numbers on a computer keyboard completely opposite to numbers on phones/ATMs etc?

The numbers on a computer number-pad are designed for easy entry of a whole series of numbers - number-crunching as it were (these are the true confessions of a former accountant!) - having the zero on the bottom is the quickest arrangement for rapid data-entry; the numbers on a phone / ATM (and remote controls) on the other hand are designed as a replacement for the rotary dial which had that same sequence, and also included the letters of the alphabet - which is the basis of the current SMS / text messaging system. You don't need to enter as many numbers on a phone key-pad, and having the numbers and letters both arranged in ascending order helps us to remember the association.

The numbers on the keyboard are designed to mimic the numbers on an adding machine as typically used by accountants

The number pad is not upside down. Take a look at your calculator. That is where that sequence comes from.

It's not upside down-- the keys were arranged that way on adding machines, way before there were such things as computers. The question to ask is why telephones are arranged the way they are, because they (touch-tone models at least) came AFTER adding machines were invented.

calculating machines were invented first, so then, the telephone num pads were arranged the opposite precisely to not confuse one with the other

OMG it's true!