The "explanation" is called "hope". If there's no signal, there's no signal. Voice will actually get through better than data (with a GSM phone and an ear that's been listening to two-way radio signals for years, you can almost understand a signal that's weaker than the noise) if there's some signal, and text will get through better than voice. But with "Emergency calls only", you have no signal, so no signal will get through. (But it's nice to hope, unless it keeps you from doing something actually useful. [I say this with 60 years of two way radio experience behind me.])