Just wanted to add a quick note to anyone having trouble dialing an extension with Google Hangout / Google Voice
You need a working microphone plugged in and active
Without it, you won't be able to dial extension
Note that the actual dial tone doesn't have to exist. For example, when dialing an extension 3000 after dialing the number 555-555-5555, you don't hear the usual dialtone or extension sounds when dialing 3000. What you will hear is a series of clicks, but the clicks will work if there's a microphone plugged in.
Hope this helps someone