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 if additional fees or data usage applies from your data provider. 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. You can make and receive phone calls with either phone number.