This is bad.
There are a bunch of dbus-signals like

com.nokia.phone.SSC.modem_state_changed_ind
com.nokia.phone.GPRS.Available
Phone.Net.operator_name_change

It took me some time to figure out
how to start the connection at the right time. The
current way is:
wait for signal
com.nokia.phone.SSC.modem_state_changed_ind = online
and wait for
Phone.Net.operator_name_change
after that I open the connection.
I have to listen for both signals because right after changing
the radio mode, operator_name_change is signaled two
times, but only after the third time (after signal
modem_state_changed_ind) I can start the connection.
I hoped this way would work for others too. But
it seems this doesn't work for all network operators.

I don't know how to find out what could be the right
way that works for all operators.