Install latest freepbx as your voip/sip server (in a vm if you like) That's a lot of function for free. However, you can then create your own user interface using b4x.
Sip is super complex, I personally wouldn't try to revert this wheel.
If operating a system connected to the public world, you must setup security else you'll be hacked within a week.
(do not purchase commercial modules, or the awful zulu mobile, Sangoma are complete idiots, I know coz I'm a reseller)