5RU20W HD desk/wall wired or wireless (WiFi) business phone. Not POE
1RU68W HD wired or wireless (WiFi) operator/executive phone.
Ensure you have a solid internet service.
Check that you have enough network switch ports and cabling. Remember these phones can share an ethernet cable with a computer, it has a 2 port switch in it.
These phones also can operate via WiFi, no need to run a cable to every location!
You will want a high capacity firewall and ethernet switch.
If using WiFi, you will want a good WiFi access point.
Expected Monthly Costs:
It's hard to give an exact quote, because taxes and regulatory fees vary for every location. If all you need is 2 lines and our standard features your monthly bill will be 59.95 plus taxes and fees of about 5 to 10 dollars, expected: $66 to $70 per month for unlimited USA calling.
For a 6 extension system, lets estimate you have up to 3 incoming and outgoing calls at the same time, and set your system for 3 lines. That puts your monthly base bill at 84.90, your total would be less than $100 per month with taxes.