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With iOS 13 and later, when you're on a call, if the carrier for your other phone number supports Wi-Fi calling, you can answer incoming calls on your other number. When you're on a call using a line that isn't your designated line for cellular data, you need to turn on Allow Cellular Data Switching to receive calls from your other line. If you ignore the call and you have voicemail set up with your carrier, you'll get a missed-call notification and the call will go to voicemail. Check with your carrier for Wi-Fi calling availability and find out whether additional fees or data usage applies from your data provider. If you're on a call and your other line shows No Service, either your carrier doesn't support Wi-Fi calling or you don't have Wi-Fi calling turned on. It could also mean Allow Cellular Data Switching is not turned on. When you're on a call, an incoming call on your other phone number will go to voicemail if you set up voicemail with your carrier. However, you won't get a missed-call notification from your secondary number. Call Waiting works for incoming calls on the same phone number. To avoid missing an important call, you can turn on call forwarding and forward all calls from one number to the other. Check with your carrier for availability and to find out whether additional fees apply.