It's currently not available for purchase and was a one-off construction made especially for the BT ad. It had three puppeteers controlling it and an ancient tape recorder was used to play the Karma Chameleon tune. There are rumours around, that due to the success and calls BT received about the phone, that they will be producing one in the near future.
To confirm, this information is on the BT website:
"The 'Karma Chameleon' telephone which features in one of our television commercials is a one-off novelty phone made specially for the commercial. However, our Marketing Consumer Products Division are evaluating the commercial prospects behind making and selling the phone."
http://www.bt.com/talktous/answers.jsp?ID=PQA_192
Raydial