Hi,

It's all undocumented, really. Qt Mobility may have interfaces for this sort of stuff, I don't really know.

Oh, if you'd like to ensure your sanity stays intact, don't use Telepathy.

Making a phone call: http://wiki.maemo.org/Phone_control#Make_a_phone_call
Searching for cellular networks: Scanning and switching networks... this took me a while but I did manage to come up with code for a person who asked on maemo-developers: http://www.gossamer-threads.com/list..._view_threaded (as per my next post in that thread, do add g_strfreev statements...)
USSD: The USSD widget etc. are open source.
SMS: Try http://talk.maemo.org/showthread.php?t=58473 and http://talk.maemo.org/showthread.php?t=36148
Oh, connecting to a GPRS AP: http://maemo.org/api_refs/5.0/5.0-final/libconic/

Then there's my 3G/2G Mode Selection Applet
and my Caller ID State Switcher Widget.

Oh, and the Call Forwarding applet is brilliant.

The Telepathy component that interfaces with these lower level D-Bus interfaces, telepathy-ring, is closed source but the MeeGo version is open. However, because MeeGo and Maemo use a different set of telephony components, don't rely on that code to learn about these lower interfaces on your tablet. Instead, use it as a starting point for figuring out the mess that is Telepathy.

Really, much of the phone stuff can be controlled via D-Bus. Look at the strings of the file /usr/bin/libconnui-cell.so.0 for the names of the available methods, interfaces, etc.

Graham Cobb has also written a most excellent wiki page on how to figure out these interfaces.

And www.bleb.org/software/maemo/telephony-maemo.c is a must read.