Wondered this myself… I'm looking to replace my current mobile phone and looking at what's on the market, DIY using the LTE version of the Onion Omega sounds like a reasonable option.
The Onion Omega LTE looks to be built around a Quectel EC25 cellular module, and looking at the schematic https://raw.githubusercontent.com/OnionIoT/Omega2-LTE/master/Hardware/R1.1/Omega2-LTE-v1.1-Schematic.pdf (page 5) I see the I²S pins are disconnected.
That said… whilst looking at a rival module, the SIM7600X, I saw mention of an "audio" port…once you feed the appropriate commands via the AT port, apparently 16-bit mono 8kHz PCM streams out the audio USB-serial port.
Knowing this, I had another look at the Quectel datasheets, found this:
Question is, is anyone able to try this with an Onion Omega LTE to confirm whether it works or not?