3 office locations - Each with different number

Hello, I Have gone in and done about 80% of the setup in the Admin portal, users, call groups, etc.... I am struggling with the setup of the MAIN numbers I will be porting in from our current company. We have 3 locations currently and each office has their own number. I want this to remain the same and not be subject to 1 main line. How do I setup one and then mirror the other two? The only changes I will have in the background would be which extensions are ringing at which location, that i understand.