First, get your measuring stick, because I doubt if the garage is that far away. That would be the length of three football fields. Second, the distance is not usually the problem, but electronic interference is. So every TV and fluorescent light between point A and point B disturbs the signal. That being the case, your best bet is a good phone system with the intercom feature. You can also get phones with intercom video.
They actually have baby monitor systems that are affordable that have audio, video and movement alerts. They are really technologically advanced now. I think they may be a good thing to research.
I know we're talking human alarm systems here, but FWIW there are hunting dog systems that go a couple miles, so there is technology to go 1, 000 ft (if it is that far).
A baby monitor would have the features necessary, but I'm telling you that the Summer brand one we have won't work out in the back yard, let alone 1000 feet away.
What might work for you would be a monitor that you use over the internet and you check it with your iPhone.
Second, the distance is not usually the problem, but electronic interference is. So every TV and fluorescent light between point A and point B disturbs the signal. That being the case, your best bet is a good phone system with the intercom feature. You can also get phones with intercom video.
Angel
What might work for you would be a monitor that you use over the internet and you check it with your iPhone.