How do you dial an extension to a phone number? Do you just dial it all together or do you have to hit pound key then dial the rest?

This is known as DID method ( direct inward dialing). In an EPBX line this facility is incorporated so that the caller can directly dial an extension number after dialing the EPBX number.
Normally,when the EPBX line is connected, the voice recorded message will ask you to dial the desired extension number after a beep.