Gira have a range of video intercoms and supporting hardware. They are a German company, so everything works offline by default. Not cheap, but it's also not going to be made obsolete in 3 years time. I believe they use SIP under the hood.
Problem with Gira is I've never actually figured out how to purchase it as a consumer. The products look amazing, but it seems impossible to buy in the US.
https://www.gira.de/produkte/tuersprechanlagen/tuersprechanl...