(1) Try and Force Restart your iPhone EXACTLY as shown below and see whether that resolves the issue:
- Press and quickly release Volume UP button
- Press and quickly release Volume DOWN button
- Press and Hold the SIDE button until an Apple logo appears and then release the Side button (Can take up to 20 seconds. (DO NOT release Side Button when invited to Slide Power OFF).
(2) If the above does not work then - Contact your carrier because the Call Waiting feature may not be functioning proper on your account with the carrier. The Call Waiting feature is provided and managed by your carrier. Explain the issue to the carrier and see whether they can resolve it on your account at their end.
Axel F.