Nokia 2115 is not a regular modem, but a cellphone which can be used with some computer operating systems as a gprs modem.
The gnokii project may be able to help, but I cannot find your model in their support list.
It is highly likely you have an unsupported phone.
Try plugging it in, switching on, count to 20, then enter "lsusb" in a terminal and see what is there.
Hmmm... I see that your model may have an issue with overheating batteries... boom?