You need to have some kind of CSU/DSU to connect the telco line to the WAN port. If you connect it without modem or CSU/DSU (internal or external) your hardware will go bad. You can use OFC but will need to talk to the ISP. As you need to connect it to your Ethernet port and he need to give you an IP address. Instead of Router you will need a switch and ISP will have to do routing for you. So you need to convince him to do that. If you are the only customer requesting this I doubt he will do.
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