Why I got the message "Could not save call waiting setting" in my iPhone 7?

• 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.

For CDMA accounts, contact your carrier for information about enabling and using these features.

Call waiting is a cellular carrier feature; make sure your cellular carrier offers it.