Button to hang up or pick up missing on my Lenny 3

I had the same problem on my Lenny 3 and in the end, you can also try checking in the settings if the call notifications haven't simply been blocked for some mysterious reason.
To do this, go to Settings then Apps, click on the Phone app and check the Notifications line; if it says blocked, you can change it so that normal appears.
This resolved my call display issue, finally allowing me to answer calls.