Maybe I'm way off base with this post, but I have seen something similar to this with 2008 and 2616 sets. It was a procedural error where a person would try to transfer, or conference, a call to another party. If they tried to bring the call back, the transfer, or conference, arrow would stay illuminated and lock up the phone. We found that if the first party would hit the release button, then the call would act like it was on hold and be able to be picked up again without locking up the phone.
Mike