The text function requires two arguments. By including the , you are recognizing the second argurment, although by leaving it blank you are probably having it default to a general format. Better form, in my opinion would be
="My name is "&Sheet2!A1
="My phone number is "&text(Sheet2!A1,"000-0000")
The TEXT() function does require two arguments, so your second example shouldn't work. Why it accepts a null argument, though, I'm not sure. There isn't a default format (at least that's documented) and you can't use the General format (in XL2K, anyway).