On the phone screen, in front of the extension number, you can see the number of calls you have missed.
Push the settings key on the phone (has a gear icon).
You should see 5 options. Choose 'Call History' by using the ring buttons to go up and down, and push the center of the center of green ring to select it.
After choosing call history, you will have a list of all the lines available to you. Choose the one you wish to see using the green ring.
Push the center circle to confirm.
You will see the list of all your calls, you can scroll down using the green ring. Selecting any of the listed calls will initiate a call back to that number.
Push the return arrow to go back to previous menus.
Alternatively, you can select 'more' to get more options, including the option to clear the list.