Connect standard SIP server

Hello,

We have own standard sip PBX server, we would like to make soft phone interface in ionic 3 and connect SIP server from the app.

I have been through various JS and modules like SIPJS, Limephone etc, but they all are integrated to the specific SIP server. SIPJS asks for WebSocket server url, how ever we have only server url, username and password. Server is UDP port enabled on 5060. How should We configure WebSocket? is that a middleware between ionic and SIP server?
is it possible to write all event handlers like Register, invite, Subscribe, Publish, Notify on own middleware server or the standard SIP server handles them. Please guide or share any related references.

Best Regards,
Zubin.