The Ekiga is free softphone also khown as GnomeMeeting. Developer's site.
1. Install Ekiga softphone from software manager.
2. Start application, then click Edit tab in top menu, then select Accounts.
Also you can open Accounts window by combination Ctrl + E.
3. Click Accounts tab in this window and select Add a SIP Account in menu.
4. Enter profile name, for example, MCN.
Registar: enter SIP-domain address from registration email
User: your SIP ID
Authentication name: your SIP ID
Password: your SIP password
Enable account: on
5. Click OK button. Close Acoounts window.
6. Your account is configured. To start a call click to the dial pad button as shown on screenshot below.
Your application is ready to use!