I am enamored with the idea of SDF, and I think it is an important part of computing history and the present. That being said, I am curious as to whether anyone actually finds it useful—aside from the fact that it hosts the instance!!
Mostly these days as a community and also a web presence. Through the years I've used it for many things: file storage, places to practice coding or run builds on other systems, a place to learn, gopher, a VoIP provider, a VPS provider, DNS host, blogging service, mail+usenet service, games, mailing lists, VPN, and a bunch of other stuff I'm sure I forgot.
I've been a member verifiably for 20 years (see my uinfo) but I've been around for longer than that. Probably closer to 25. It's always just sort of been an extension to my computing. My membership has waxed and waned but there was rarely a time when I wasn't donating at some level.
I find I don't rely on services on SDF for critical stuff because sometimes they go down for extended periods of time or they go neglected. I do, however, appreciate the hell out of it.
OpenVoIP on anonradio is fun. Dial into a party line extension on the air.
SDFers can get access to a SIP extension. I think it's enabled by way of the maint command from the shell. From there, you can use a softphone like linfone to communicate with other SDFers.
The SDF VoIP service is also connected to the C*NET telephone collectors gateway and you can dial around to explore that network.
I paid for a DID for a while and connected to that with a grandstream DECT base station with a few wireless phones as well as a grandstream ATA and an old western digital phone. For a long time that was the only phone service in my home.
Look at the join and tutorials pages on sdf.org for all sorts of other info. There's countless services on the fortress. Some old, some new, all in various states of maintenance. Keep digging. You'll find some cool stuff.
I currently run my landlines at home over SDF VOIP. Now that I can get a local number I'm going to add on a DID. Currently I dial the main switch board and punch in my SDF ext to call home. I love how I can sign up for so many diverse offerings in one awesome community.