office networking help routing ethernet and phone

Would greatly be appreciated

Main building has all the incoming phone lines and ethernet.

Building main - line 1,2,3,4 they all go to RJ16 phone jacks - internet - router - 48 port switch - 48 port patch panel - lines run underground to all out buildings.

What I want to do is be able to use the existing ethernet lines running from building the building to extend the landline phone network without having to run phone lines and digging up the ground to go from building the building.

In the out buildings I would like to use IP phones and set up an internal phone network also. But still be able to use the standered coper phones in the main building. Also if possible I want to use coper along with the IP phones. But I want it so you don't have to dial 9 to access an outside line would prefer it to automatically selects the next free line. We have 4 hardliners coming in and our internet is satellite so a VoIP option will not be optimal due to very high pings.
We are working with Telus to get dedicated fiber optics but even tho they have fiber going down the main road it's gonna take them a couple of years to get the permits. Even tho I'm paying full cost to get 10Gigabit fiber. For now I just need some help on figuring out what I can do for the time being. Maybe a PBX system that can connect classic coper landlines to IP phones. Also will be using a fax machine also.