Sagem DMC830 Dect+GSM phone question

Hello,

Does anybody know how Sagem DMC830 dual mode (dect + gsm) phone works? I want to buy this phone for my wife to use both as mobile phone and as home cordless phone. But I am afraid of the following problem. My wife, being at home, is calling to her mother. In this case phone works as dect, so our home phone line is being used. Let her speak, say, during an hour. I am somewhere outdoors, and want to call to my wife (using my own cellular phone). If I am calling to my home phone number, I expect it to be busy since line is in use. But what happens if I am calling to my wife's mobile number? Will she hear the ring? Or she will have no way to notice that I am calling, and I will hear "out of the coverage" message?

Thank you.
Sergei Markelov