Using legacy Nortel phones with Asterisk?

We are looking for a solution to allow us to use older Nortel M7310/7308 sets with an Asterisk box. I have seen the Citel, but it is way out of our price range. I would have thought this would be a common item, especially given the huge number of these phones out there, but it seems not. Any help is appreciated, we don’t want to replace all the sets and run new wiring.

AFAIK Citel do the only solution.