This is probably a stupid suggestion, but better to cover the bases than assume it . . .
have you double-checked that the number you are dialling is valid for the ISP your sister is using? Modem seems to be working fine, but not connecting, which is usually, but not always, down to user error in inputting the phone number.
have you double-checked that the number you are dialling is valid for the ISP your sister is using? Modem seems to be working fine, but not connecting, which is usually, but not always, down to user error in inputting the phone number.