For my situation, as a backup/secondary VPN? I haven’t really noticed any downsides. It’s come in handy couple of times when I messed up the wireguard config on my wireguard server - it’s on an oracle cloud VM and it’s harder to get a local terminal in there in case you lose SSH/remote access, which I did.

If you’re already running wireguard and just want a VPN, there isn’t much that you’re missing out on except for convenience when it comes to device management and routing, automatic hostname DNS resolution, and also getting access to more advanced features like meshing and failover LAN/subnet sharing without needing to figure out how to do it in bare wireguard.

Honestly though, it’s free and makes for a great hassle-free backup VPN that just works. I use wireguard as my primary because it’s fully self-hosted, runs at the kernel level instead of within the userspace so it’s faster, and is more native than installing third-party solutions; with that said, I still run tailscale on all my servers as well in case I bork something while editing wireguard configs at any point.

I don’t think there’s a ready-made solution for something like this. If you were going with the coding route and we’re just concerned about making calls and not receiving them on your main phone, then you could verify your second number with a SIP carrier like twilio so that you can use it as an outgoing caller ID with them, create a sip trunk with them, spin up a local asterisk server and hook it up to the sip trunk, and then program your asterisk dialplan to allow you to dial out using that second number as the caller ID. Then you’d just use an app like linphone, sign in using your asterisk credentials, and dial out from there. It’s a lot of setup to do if you’re not familiar with asterisk or sip trunking though