You've probably worked it out by now (!) but for anyone else with the same issue:
- Ensure that the group you have added your extension to is the group that the incoming lines are set to ring on, as there are 20 groups on the system.
- Make sure that there are no diverts set on the group and that auto attendant is not switched on.
- Make sure that you can receive internal calls on the extension, so make sure that nothing like do not disturb is set.