RTP pager accepts unicast/multicast stream encoded with G.722 or G.711a/u and should be compatible with most VoIP phones with "BLF" keys.
Receiver for https://github.com/duncanthrax/scream accepts unicast/multicast L16/44100/stereo stream from virtual sound card running on Windows.
Both applications are using external I2S codec (PCM5102).
Issues: packet loss (less visible with RTP, perhaps because of lower packet rate), varying depending on access point / channel / transmission mode (unicast might be more reliable than multicast). Jitter buffers are configured as quite long, so at least short messages should be received without buffer underruns.