It turns out that your iPhone can use only one cellular network at a time when you are on call. During a call on one line, your other line shows No Service. And that's what it is supposed to be like!
However, it is still possible to answer incoming calls on that number. It's all about Wi-Fi Calling. Check with your carrier if this feature is supported on your device and if it is on.
Tip: When you're on a call that isn't your default data line, you need to turn on Allow Cellular Data Switching.