Well, today I found my phone off the hook also, right next to the cat food dishes on my counter top. There must be a cat conspiracy afoot!
Seriously, I think my phone got accidentially knocked off the hook by Seven (the athletic one) jumping from the counter top to the top of my "bridge" across my living room (a favorite cat place). The phone was just in the way of a slightly ill placed rear paw, probably.
However, both Seven and Cassie will knock pens or pencils off the desk or counter top just to see them fall. They must enjoy that! And they will do it repeatedly.