Intercom use within a shared line

We have a customer who has a single shared line with 8 phones. They need two way communication (intercom) internally on their shared line. Is there anyway we can get internal shared line intercom functionality? There are 8 phones in this group with a total of 4 lines. If you think about the way landline phones work if they pickup a phone on line 1. Then a second user picks up another phone selects the same line. That functions as a two way communication with both users being able to talk on that line similar to how intercom would work. How can we setup two way communication amongst these phones? Intercom is based on the function calling out to a different extension.