Back-to-Back UA architecture?

Hello developers!
Can I implement VoIP SIP (only audio) based on jitsi, directly and only between my client and manager?
Between clients this should be impossible. Only a manager can call clients, but at their request. Manager based Windows. Clients based linux Yocto.
Can I use only client programs or do I need a server? If you need a registration server, can I organize it using your application? I should not use the registration server outside. Only inside my company.
Does this require Back-to-Back UA architecture?

Is there a sip-client jitsi jar-file for yocto?