after the welcome message, dial the room number and press the #
on Skype, activate the dialpad to dial the roomumber or other functions
The current version of Skype on Linux doesn't have a dialpad anymore, so you can't enter the phone conference with this software. Either downgrade Skype or use the regular phone.
Good news: The dialpad is apparently only hidden behind a keyboard shortcut: type "d" and it should appear: [1]