A leased line requires a special setup of your S0 interface. After that, you can not reach any other destination than the one the leased line is set up for. It's also rather expensive.
A flat rate is still a normal dialup, therefore the setup should be done like any dialup connection. The only difference from a normal dialup is the pricing. See section dialout. Also please note that the connection on a flat rate will usually be stopped by your internet provider if you stay on for too long - so you can not rely on being online all the time, if this is what you desire.
The data is 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 and no. You can configure HiSax for both at the same time, however you can only use one of them at any point in time (you have to switch off the leased line before dialing out). It may work occasionally simultaneously, however the driver has not been written for it so the results are not deterministic. Also make sure that you use the correct channel.