If you've got good enough wired internet, a voip based "landline" is pretty inexpensive, and if you run with the G.711 codec, it's basically landline equivalent, with a small latency penalty (much less than cellphone though).

In theory G.722 (AMR-wideband, aka "HD voice") is coming to cell phones via VoLTE and is also available (in theory) via VoIP providers, and should be better than landline quality, but with added latency. Of course, once enough steps have added latency it becomes noticeable, but perhaps the increased frequency range makes it worthwhile; and it requires interop work between carriers.