First of all, what do you (or they) mean by "Business Trunking"? That makes no sense by itself.
Good News: (Re)Search for cloud PBX hosting. It will be the better solution for you.
Bad News: Your Panasonic will have issues with most newer services. You may need to buy add-on cards to be able to connect, but even then because it's an older system it only has simple/limited VOIP functionality.
If you choose IP Centrex, technically it can be done, but it will be a challenge and depends on if the provider supports your model and the needs you have. Eventually,
you will probably end up using a SIP gateway and even then it will need a LOT of testing.
If you choose SIP... You will need an extra card (KX-TDA3450 I think) but you won't get any modern functions like call recording, advanced call routing, or integration with UC tools. On top, you may be limited when it comes to codecs and bandwidth management and
you will probably end up using a SIP gateway.
As you see, regardless of the solution, you will probably need a SIP gateway to make the Panasonic work.
A SIP gateway acts as a translator between the Session Initiation Protocol (SIP) used in VoIP communication and other signaling protocols, like those used in traditional phone networks (PSTN/ISDN) or other older VoIP systems
Honestly, it would be better to look for a modern PBX and subscribe to a cloud PBX provider.
There are a few workarounds that may or may not work for your particular system, but let's be honest, this is a task for someone who can A. Be there and have hands on the devices, B. Has experience and knows of those things, so better contact a local business that can consult and execute the upgrade, rather than asking on forums
Best of luck.