English Pronunciation: Oh and O

I'd have called you if I'd known you were in town.

I didn't know you were here, so I didn't call — but I would have.

1. I'd have called youI would have called youwould have + past participle: something that didn't happen.
2. if I'd knownif I had knownhad + past participle: an imagined past condition.
3. you were in townyou were visiting here“In town” means nearby for a while, not necessarily in a town.