I'm guessing it's about 1-2 months away. The hard bit is that we need to effectively have Element Call share the same matrix client as Element itself does, in order to embed Call into Element effectively. We call this matryoshka embedding, and it's looking promising to get it working in the next week or two, and then will need to actually update all three Element clients (Web, Android & iOS) to support it.
Obviously folks can also just click into a standalone Element Call to make it work (plus Hydrogen is also adding native Matrix calling).
Element Call doesn't currently need any custom server - it just uses your existing homeserver; Synapse, Dendrite, Conduit etc. However, it only scales up to 7-8 users - to go beyond that we'll need to add a SFU, which we're also working on.