Part of my problem is I don’t know what the feature I want is called so it makes it tough to search for it. 
Keep in mind this is for a home system and if I’m going to VoIP I have to keep the wife who is NOT technically savvy happy
Here’s my scenario so far, I’ve loaded FreePBX onto a Raspberry Pi, connected 3 extensions (Cisco 7960 phones), extension #'s are 701, 702, & 703, I can successfully call from any extension to any extension. I have setup Google voice (I understand its going away soon) as my test carrier and can successfully place and receive phone calls. Incoming calls go to ring group 600 (?) and ring all the phones, I can then pick up the call from any phone without issue.
My problem is, if say I’m on the phone (station 701) and my other half wants to join into the conversation (from station 702), how do I set the phone up that all she has to do is simply pick up the phone and select the line with the existing call?
In other words I want it to work much like a dumb phone were any phone can pick up the call if needed/wanted.
Thanks,
Jeff