Bear with me here, because I know this is going to sound crazy. And yes, Even though I have had experience with SIP, I don't think I have ever had to deal with what I am about to ask now.
Lets say you have 2 polycom phones sitting in a box, brand new. Lets then say that some people like these phones so much that they think it might be a great idea to get more of them! Pretty cool, right? your first major project that was a win was getting rid of their super old Shortel system and moving to the same provider as your ISP. From 100 desk phones...to 5! thats a cost savings, right?
Well, We have 4 desk phones that are actually in use on the ISPs VOIP Platform. this is all good :)
One day, someone comes to you and says "Hey Flat-Entry90, I like those phones. I'd like to be able to use one kinda sorta like an intercom! you know, I'll have one in my office, and joe-schmo will have one in materials office, and Karen will have one in HR. I want to pick up the phone, hit the button, and then have the other phone ring" And now 2 un-opened polycom phones are on your desk.
I understand that SIP is needed to facilitate calling, but I'm trying to find a way to see if "intercom/walkie-talkie/phone to phone on the LAN" mode is a thing, or a way to say....ehhh "nahhh gotta pay the ISP more for another user".
I was thinking I might be able to make something work via Teams, but I cannot get a license for additional features/ direct routing.
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I hope this was understandable.
The description your user gave was one phone calling another phone, but then you described it as "intercom/walkie-talkie/phone to phone on the LAN" which isn't the same thing, so I'm a little confused. If they just want to be able to make calls from phone to phone, you can either use direct IP calling or get a cheap PBX, the intercom/walkie-talkie would be called paging, which is something else entirely and not usually done through phones but through paging interfaces.