Miguel:
I did a search on www.google.com with the keywords:

mitel MT8880 datasheet

... and located the full data sheet for the MT8880 part. Here is the web adress for the data sheet.

http://www.ee.mut.ac.th/datasheet/mt8880c.pdf

I used this part about 12 years ago in an 80C51/80C31 design where I was detecting what was being dialed on a phone line. I was able to get it working nicely with the information in this data sheet.
Good luck
Mike