How to setup one main number with multiple extensions?

We moved from a antique Cisco VOIP system to RingCentral. We have a satellite office that piggybacked off of our Cisco system and had a local number here. In Cisco land we had the satellite office setup with 1 number that had multiple extensions under it. How do I replicate that in RingCentral? I’m running into a wall trying to get it to work. I set up the second site and when I try to add a number from my inventory to the site I do not see any of my available numbers.