If you’re looking for a softphone to use with Asterisk X-Lite is great. It works on both Windows and Linux, although the configuration screens are a little different on the different versions.
All you should need to get it working with Asterisk are the following settings (screenshot from the Windows version) –
Obviously you’ll need to change the server name to your Asterisk server’s DNS name or IP address, and the username/secret settings to the extension you created in Asterisk.
Please make sure you pick a difficult-to-guess secret/password, especially if your box is accessible from the Internet.