Sccm is too big. I don’t think you’ll find an all in one alternative.
Piecemeal is easy though. Wazuh for inventory. Chef or ansible for packages and desired state config.
Lots of pxe boot deployment options, including wds, which sccm pxe is built on.
Maybe tackle one feature at a time and pose your questions one at a time, so people doing pxe boot imaging can chime in on that specifically for example.
I ran FreeSwitch with a signalwire sip trunk. I don’t think there’s much cheaper than that. Pay as you go, less than a penny per minute. Only monthly fee is holding onto the numbers you want, each number is like $0.50 per month.
But installing and maintaining it is a challenge for even good sysadmins.
I’d go with 3CX or a similar all in one provider if I had to make the call.
Also, use a good USB headset and a soft phone on your computer instead of that old cisco handset. Get away from handsets ASAP. They are the most expensive thing, vendor support dries up fast, and the audio codecs go stale. Firmware vulnerabilities emerge, compliance requires you to update them, so every 5 years, new handsets for everyone. It’s a joke. USB headset or a good USB speaker phone will work with any system, until they physically break. 3cx has soft phone clients so handset isn’t needed at all.