If I understood correctly your question (and your set-up)
You might be able to have Zulu working, but there are two things you will require:
a) A valid ssl certificate for the server (specially if you need to use mobile) and for that you might need to make your pbx to face internet (if you will use letsencrypt) own certificates won’t work.
b) Your devices will need to be connected to your private network (I guess is a vpn) Zulu does not include a VPN client, but if your device (mobile or pc) can reach that private network, it should work.
I have a set-up like that with letsencrypt for a and openvpn for b, feel free to contact Sangoma support if you need assistance on this.