I am setting up a fresh server using AsteriskNow and got a few of these Cisco phones to evaluate. I was attempting to test call pickup (**) and would immediately get an "invalid call" error on the phone screen and a pulsed busy signal on the phone. I tried the same from a different kind of phone and it worked just fine.
After some digging I found that these phones have a built-in dialplan string that comes factory-default with a bunch of garbage in it. I changed it to the following and it began to behave: