I don't understand what you mean by "locking" and "unlocking" the modem.
If the PC has a "dialup" modem installed, you don't need to install any additional software to create a dialup connection. All you need are your account Username, Password and the ISP Phone Number. Run the network wizard and follow the prompts to create a new dialup connection.
Try creating a new new connection and see if that is more reliable.