Can you please check the below reference if this answer your question Faq no 13:

https://support.ringcentral.com/article/5940.html#faq13

This is not a programming nor an app development issue. Let me move this to the Customers community so you can have better conversation.

Okay well I need another customer on here to tell me how to setup a shared group to intercom or speak on a line. I have extension 1 as the shared group extension. There are 7 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. If this is a community question for other customers there must be a way to do this that another customer here knows how to do. I would be greatly appreciative of this if someone here could tell me how it can be done. According to all the tech support representatives I have spoken with says it cant be done. If this isn't for the development side then that means the capability must exist already. Any help with this would be so greatly appreciated.

HelloCraig, apologies! Our tech support is correct; this can't be done. I would highly appreciate it if you post this as an idea? Click HERE for our feature request portal. Thank you!