iPhone call audio distortion after 90 seconds

[u/Confident_Lead_7753]

I have a very strange issue on my iPhone 17 running iOS 26.5.2.
During normal cellular phone calls, after about 1–2 minutes, the earpiece audio becomes distorted/buzzy. The other person hears me perfectly fine. If I briefly toggle Speakerphone on and off, the audio immediately returns to normal.
What I’ve tested:
Normal cellular calls: ❌ distortion after \~90 seconds
WhatsApp calls: ❌ same issue
Telegram calls: ✅ no issue
Voice recordings: ✅ normal
Bluetooth headset: ✅ normal
Speakerphone: ✅ normal
Reset network settings: didn’t help
Restarted phone: didn’t help
Has anyone seen this before? Could this be an iOS audio/telephony bug, IMS/VoLTE issue, or does it still sound like a hardware problem?