The data are simply sent out! Other than a ping, there is no way to find out whether the D64S or 2MB line is up or not. Only S01 or S02 lines have a D channel and have something to use with signaling, however the best known solutions also use this 16kb for data transfers to get 144kb instead of 128kb (i4l can only to 128kb).
A later version of the new HiSax driver supports D64. Configuration is normal with the following specialities. HiSax has to be run in leased mode:
/sbin/hisaxctrl HiSax 5 <channel>
/sbin/isdnctrl bind HiSax,<channel>/sbin/isdnctrl eaz isdn0 1 /sbin/isdnctrl addphone isdn0 out 2 /sbin/isdnctrl addphone isdn0 in 3
Use a later HiSax version. First initialize the ttyI* device you want to use with "AT&E0" (set usage of first B-channel) and "ATS0=1" (autoanswer on first ring). Then set HiSax in leased mode:
/sbin/hisaxctrl HiSax 5 <channel>
Yes, you can. But you have to make sure that you use the correct channel!