If you want port forwarding and can’t replace or control the router you need to ask the phone company to give you access to the router or setup port forwarding.
Other ideas:
Can you ssh to the PBX? Then you may use SSH tunnel port forwarding.
What about software like ngrok or pagekite, see Server access behind Mobile network using ngrok or pagekite
You may use remote support software like Teamviewer to connect to a PC in the phone network.