Networking Phones Together in Building
You will need a phone switch to make this work. I think these phones use a Siemens HiPath switch. The phone line from the phone company connects to the switch, then the phones connect to the switch. The switch will require detailed programming. You can probably find one cheap on eBay. However, I don't recommend this path.

Instead, I recommend buying an "expandable business phone" from some place like Office Depot. I know RCA makes a decent line, and that AT&amp;T has a competing product but I have no experience with those. Unlike the OptiPoint phones you have, no switch is required. The phones talk to each other over the wiring you get from the phone company, but in such a way that phone company doesn't think you're making a phone call through them.

To make this work, you just have to get the phone company (or a wiring contractor) wire all of your phone lines (or just your single phone line) to every desk. Then plug the phones in. Any phone can answer any line, you can make internal calls, you can make conference calls, and you can transfer calls, just like you would with your OptiPoints.