I'm going to echo a couple of other comments here about the Unifi stuff. It's awesome.
I have a Synology NAS, which comes with the Surveillance Station software on it. Unfortunately when I bought it, the description on Amazon didn't say anything about having to pay for licenses if you want to use more than 2 cameras. $50 per camera. That's a ripoff when you look at Blue Iris and some of the other software out there.
Since I already have some Ubiquiti network gear, I upgraded my Cloud Key to the Cloud Key Gen2 Plus. It's got a hard drive in it (upgradable), and a build in NVR. It can support up to 20 cameras. The rub is that you have to use Unifi cameras. I bought the G3 Flex cameras, 4 of them. This thing is awesome. The phone app works really well, and the motion detection is great. The timeline view even has little thumbnails of the motion that you can click on and it will go to that spot. Also, if you scroll through the timeline, the video speeds up so you can review quickly. Much better than the Synology stuff. I record everything, 24/7. But the motion detection is great for finding out what I need to look at. The cams record sound also. No licensing fees, no cloud service fees, nothing. You buy it, you own it, and you have full control over your video content because it's local to you (but it's accessible through their free cloud connection if you're not at home).
There are a bunch of Unifi camera options. One of them has 2-way audio, but it's indoor only and I've been unable to verify if the 2-way audio works with the Unifi Protect app, or if it only works with their older NVR app.