I built an interface at one time to a Nokia phone using an 8051 for MBUS. It's just an open collector single-wire TTL level (OK a little lower voltage) at 9600 baud, 8 odd 1.
It was a few years ago, so I don't know how current that is. Here is as good a link as you are going to find for this stuff:

www.flosys.com/tdma/n5160.html
http://samba.org/ftp/unpac.../nokia.txt

See also:

http://www.panuworld.net/nuukiawor...es/faq.htm