OpenVPN Private connection

Hi I am using a GSM mobile SIM router to host my IP-PBX using Raspberry Pi 4. System works fine internally within the SIM router. But when remote extension trying to log in to my IP-PBX, that's where problem starts. After digging, only to realized that WAN IP for my SIM data provider has multiple LAN IP. My mobile SIM router is given a LAN IP and a shared GSM WAN IP instead of dedicated one, therefor inbound traffic is not available. I need something like. OpenVPN sits in the middle between my remote extension and my IP-PBX. OpenVPN comes with a dedicated WAN IP where my remote extension and my IP-PBX can report to. Inside OpenVPN, OpenVPN would be able to connect them together using LAN IP. Scenario would be, 1. IP-PBX log on to OpenVPN using OpenVPN dedicated WAN IP. OpenVPN will assign a local IP [login to view URL] to IP-PBX 2. from my remote extension, I log in to OpenVPN WAN IP and also OpenVPN assign a local IP [login to view URL] to my remote extension. 3. When I make a call, from my remote extension, OpenVPN will send calls to [login to view URL] thus reaching my IP-PBX. The objective is to able to find and reach my IP-PBX where traditional method is not possible.