VitalPBX is an Asterisk-based phone system with a lot of upgrades and add-ons! It can be run as a single tenant PBX or with an add-on, be converted to a multi-tenant PBX system supporting all of your customers. VitalPBX also includes a soft phone (VitXi) that works great for remote workers!
Download VitalPBX here: https://www.vitalpbx.org/pbx-system-download/
View the Voxtelesys guides here: https://voxtelesys.com/tutorials/vital-pbx