Use ,
For me, I just put a comma after the phone number. So if its an automated system, and I know I can type the extension after a few seconds you can just put a comma (,) after the number and it will pause before entering the next set of numbers. I've actually done this to automate calling customer support lines etc once I know the menu structure.
Aka: (xxx)xxx-xxxx,,xxx with the last xxx being an extension and the ,'s creating a two second wait.