No audio on cellular calls (earpiece, speaker, or Bluetooth) but works on WhatsApp - iPhone XS Max

My iPhone XS Max has no audio (input or output) during ordinary cellular calls. This affects both the earpiece and the loudspeaker. Strangely, audio works perfectly fine on WhatsApp calls, Messenger calls, and Voice Memos.

Here is what I have tried/verified:

- **Speakerphone:** Still no audio during cellular calls.

- **Bluetooth:** Connected headphones also have no audio during cellular calls, but work for music/third-party apps.

- **Voice Memos:** Working perfectly (microphones are fine).

- **Modem Firmware:** Visible in Settings > General > About.

- **Panic Logs:** Zero 'panic-full' entries in Analytics Data.

- **Resets:** I have already performed a 'Reset Network Settings'.

- **Physical Symptoms:** The phone has been getting very hot recently, but has not been dropped.

Since the hardware works for third-party apps but fails for cellular audio routing even over Bluetooth, I suspect a logic board or Audio IC issue, possibly related to the "sandwich" board layers. Are there any other software fixes to try before seeking a microsoldering repair?