From some hotels (e.g., while travelling overseas) telephone switchboards generates a tone that sounds like an engaged signal until a 0 is pressed. With this one can not dial up as the ppp chat script always finds "NO DIAL TONE" and fails. A fix might be:

Replace ATZ with ATH1. May need to extend the time waiting from the 100 (1 sec) standard setting to 250, so the modem doesn't send the ATDT(phone no.) too soon.