The only thing non-standard is the way they verify caller IDs that are not yours. You can get started without that capability – just set up your pjsip trunk with Authentication None (which implies Registration None). Set SIP Server to the IPv4 address they gave you and Force Trunk CID with your main number. If they send calls from multiple IP addresses, set Match (Permit) to a list of those addresses.
Once you can make and receive calls, try turning on Generate Diversion Headers in Advanced Settings, and set From User blank. With luck, that will allow sending e.g. the original caller’s number when forwarding to your mobile. If that doesn’t meet provider requirements, you can set up a hook to send the PAI or Diversion header they want. See