Integrate SIP phone

Hello,

I would like to ask for help with the following.

We are using Capacitator to develop an application in which we need to integrate a SIP phone solution.
Essentially we will only have incoming calls (audio/video) because the intercom phone will ring in the application.
What SDK / plugin would you recommend and how would you keep the client registered (even in the background)?

Thank you very much in advance.

Regards.,
Gabor