There is no Wi-Fi calling on N5. I think what OP thought is that N5 will go into Wi-Fi calling mode like other T-Mobile phones. It doesn't. It's not a T-Mobile phone. Hence won't contain T-Mobile proprietary Wi-Fi calling module. You still need connect to T-Mobile cell tower in order to receive calls.
When you miss these calls is there a history of the missed call on the phone? If so, it's the same bug I linked to above (where the phone doesn't realizes someone is calling it). If there is a history of the call then this is something else.