If you just need to get audio into the phone, this goes between the phone and handset and allows you to inject audio from a mixer or other analog source right into the phone:

[Link: jkaudio.com]

Plug a mic into a small mixer for level control and then the output into that and you are good. But if the person speaking needs to listen via speakers to the people on the call, you will need something more expensive like this so there is no feedback on the speakers:

[Link: jkaudio.com]