=> Installation of OpenSIPS incl. mediaproxy/RTP-proxy on an existing Debian linux-server
=> Configuration of OpenSIPS as follows:
IP-PBX (firewall-protected Network) <-> OpenSIPS as SBC <->
SIP-users via Public internet
All users should be able to register from somewhere in the world on the IP-PBX. Therefore, the OpenSIPS should receive any REGISTER/OPTIONS/INVITE/...-command, fix any NAT-issius, forward the REGISTER to the IP-PBX and vice-versa. The RTP-stream should be routed via the OpenSIPS/mediaproxy-server and not directly between IP-PBX and SIP-users.
Note: It is strictly important that (of course except from rewriting IPs/Ports etc. for NAT-handling) all SIP-Messages and all Headers will be forwarded between SIP-users and IP-PBX without any modification.
Therefore, it is not a solution to use eg Asterisk instead of OpenSIPS.