The first result I found on Google for this lists a few options. You would need to find one that’s also able to pick up rotary dial clicks and convert them into the right SIP commands. It would not be hard to make a Raspberry Pi do something like that, but maybe someone has already done it and you can find it. I’m going to do this myself if I get one of those rotary phones. If you’re being really cool, you could probably install the entire thing inside the phone, and power it by POE, and end up with a rotary phone that you plug your Ethernet into.
Edit: some more googling found a forum post that lists pulse-dial compatible analog telephone adapters. You can keep your rotary phone!
