With SIP they can send you as many calls as you have bandwidth for down the same trunk. In your FreePBX configuration under Trunks, you will define a trunk for Windstream and specify 24 simultaneous channels. In your Extensions config, you will define all phones connected to the system. Then in the Incoming Routes you will define a route for every DID number you have coming in. This is where you will define your routing. Create a route with the DID number of 865-555-1234 and point it to the first receptionist. Create a second route with a DID of 865-555-4321 and point it to the second receptionist. Now everycall coming in with 865-555-1234 will go to the first receptionist and so on. If in the Extensions you define a voicemail for the receptionist position, then calls not answered will route to the voice after the ringing times out.
Obviously this is a very high level over view and if you need help with the details let me know. There are companies out there that can do this configuration for you as well. Ours is one of them.
