The interface is indeed for a digital phone. But you can call up and talk to the engineer there. He is a pretty sharp cookie and may be able to recommend something.
If not, Several people make excellent interfaces for the handset that have both balanced and unbalanced audio inputs and outputs.
Try the THAT-2 for one.. I got mine from Mike Sandman's Telephone in Chicago via Internet. It works well with Nortel phones, though I have not tried it with a receiver on an I series phone