The Kicad schematic PDF appears at the end of the first post and is named Telephone_Intercom_v0.04.pdf.
The board files are very specific for the components I used, some from Ebay/Aliexpress with all sorts of footprints so I guess these won't help much. I'll attach everything but you may be better off starting from scratch.
A lot of design complexity (hardware and software) went into maximising battery life. If that is not an issue, say you are using separate power supply, then there is a large scope for simplification for example, use a Nano classic instead of the ATtiny and keeping the boost converter running while the control unit is switched on.
Really, you should regard the project as simply a set of ideas and guidelines for someone with experience also of troubleshooting and not a set of instructions for absolute beginners. Too much is dependent on the exact components used especially the phones.
I suggest that once you have acquired the phones then assemble enough components on a breadboard to get them to ring then proceed from there.
If you have any questions, ask in this thread, not by PM so others may possibly benefit.
Kicad files: (Note that the board design does not exactly match the latest schematic!)
Telephone_Intercom_v0.04.zip (300.6 KB)