If you are using a standard phone line for your broadband interent connection, you need a wireless router with built-in ADSL modem. The ADSL modem part will replace the USB modem and the router part will allow other computers to connect by wire or wirelessly. The computer located near the phone socket can by connected using a network cable to the router (I am assuming that the computer has an ethernet network connection). If your second computer does not have wireless capability the simplest method to add capability is to buy a USB wireless dongle.
There are many models available but examples of what is available are:
USB dongle - click here
ADSL wireless router - click here