I wouldn't have called you if it wasn't important. It's about Subjunctive Mood.

These would all be clear in meaning in everyday English but two wouldn't be grammatically perfect. The following are fine: I wouldn't have called you if it wasn't important. I wouldn't have called you unless it was important

Don't take my word for this, but I think it would go like this: "I wouldn't have called you if it hadn't been important" if the important situation and the phone call have already passed. "I wouldn't call you if it wasn't important" if the situation is still there and you are on the phone at the same time. "I wouldn't have called you if it wasn't important" if the situation is there and you've already called the person. "I wouldn't call you if the situation hadn't been important" if the situation is over and you are calling the person. As you can see, it depends on the situation.

I wouldn't have called you unless it was important(reference to past call) I wouldn't call you unless it was important(reference to calling in general or future) I wouldn't have called you if it wasn't important(reference to past call again) I wouldn't call you if it wasn't important(in general or future again). I wouldn't be calling you if it wasn't important(now)