Cannot save call waiting setting

Greetings chandan1303,

Call waiting is a great feature and you've come to a wonderful place for help! To confirm, are you follow the steps listed below to adjust this setting? If not, make sure you're following the steps listed below as they're specific to adjusting this setting.

1. "Go to Settings  > Phone.
2. Tap any of the following:

• Call Forwarding:  appears in the status bar when call forwarding is on. You must be in range of the cellular network when you set iPhone to forward calls, or calls won’t be forwarded.
  • On models with Dual SIM, choose a line.
• Call Waiting: If you’re on a call and call waiting is turned off, incoming calls go directly to voicemail.
  • On models with Dual SIM, call waiting works only for incoming calls on the same line, unless the other line has Wi-Fi calling enabled and a data connection is available. See Set up cellular service on iPhone."

Set up call forwarding and call waiting on iPhone - Apple Support

If you're following those steps, did you have the opportunity to restart your iPhone and update iOS as directed by the related article? If not, restarting and updating are great steps that can help resolve many issues. For reference, those steps are listed below.

1. "Press and hold the side button until the power-off slider appears.
2. Drag the slider, then wait 30 seconds for your device to turn off. If your device is frozen or unresponsive, force restart your device.
3. To turn your device back on, press and hold the side button until you see the Apple logo."

Restart your iPhone

1. "Back up your device using iCloud or your computer.
2. Plug your device into power and connect to the internet with Wi-Fi.
3. Go to Settings > General, then tap Software Update.
4. Choose the software update that you want to install. You can choose to stay on iOS or iPadOS 15 and still get important security updates. Or you can choose the option to upgrade to iOS or iPadOS 16.
5. Tap Install Now. If you see Download and Install instead, tap it to download the update, enter your passcode, then tap Install Now. If you don't know your passcode, learn what to do."

Update your iPhone or iPad

Let us know how that goes!