I’ve been running a self hosted PBX for a couple years now. There are certainly other options out there for free, but most of them are based on asterisk like freepbx (I do not recommend that one). I’ve had success with Issabel, Wazo, and recently switched to VitalPBX (about as easy as it can get). Asterisk systems can be extremely complex, but at the same time with a friendly user interface can be set up relatively simply. I have not ventured into the freeswitch or 3cx worlds, but have heard very good things about both.
The most basic fundamentals you need to understand once you install a PBX are setting up a SIP trunk, then setting up inbound and outbound routes (at least one each direction), then finally adding extensions. Your SIP provider usually provides some instructions for the trunk settings and sometimes troubleshooting assistance. I’ve been using VoIP.ms and highly recommend them for cheap but reliable service (US and Canada). What’s nice about them is they have kind of a hosted PBX included in the service, so you can actually connect directly, and even set up failover so if your PBX goes down, reroute calls to another number or voicemail (they host that).
So for me, I pay $0.85/month for my DID plus $0.01/minute. It doesn’t get any cheaper than that besides the “free” Google voice.
If you host your own PBX, then you can freely do really cool stuff like voicemail to email, IVRs, ring groups, scheduling, and more.
I don’t think any of this has any bearing on nextcloud, nor would I recommend even going down that path. It’s a completely different beast and in my opinion not something that should be an add on app because of its complexity (under the hood).
Regarding hosting, you would probably want to pick a host that specializes in PBX hosting, or run your own. I just run a VM on ESXi, but people also do this on raspberry pi.
I hope this is helpful. A good resource I’ve used is nerdvittles.com, although I would recommend building a vanilla system and not using their mega “incredible PBX” scripts because they include lots of stuff you’ll never use. If you’re going for free, easy and reliable I’d recommend VitalPBX.
If you’re still planning on using your Google voice number for all of this, it’s not going to work (or not easily) and just remember they will always be recording everything that goes through their systems even if you do your own voicemail. Plus if you get it working they can change their protocol at any time, like they just did not long ago, breaking many setups like this. So like I said, VoIP.ms is really good but there are many other sip providers out there that have good features and are cheap. I think another good option is flowroute but I haven’t tried anything else.