I was wrong.  It turns out that the ISDN line we have can support
simultaneous calls!

I tested with my cell phone and my land line.  I tested incoming and
outgoing.  It seems all combinations work: two incoming, two outgoing,
one incoming plus one outgoing.

I don't know how many concurrent calls the line can support.  May want
to ask the phone company for info.

This means we have been overly conservative in scheduling auto-dials:
we could have scheduled overlapped calls (although I still kind of
prefer non-overlapping calls, at least for now, to make things simpler
for me).

The code, as far as I know, has no problem supporting simultaneous
calls.  But the fact that we haven't done concurrent calls has made it
easier for me to fix bugs.

The reason I thought concurrent calls were not supported by our ISDN
line was three-fold: one was just silly intuition; the second was the
fact that Pratyush told me that our BSNL line had only one number, as
opposed to Airtel's offering, which gave two numbers; the third was
that I actually saw concurrent calls failing before.  But now I know
the failing was due to other errors I made---not due to our ISDN's
inability to do two calls at the same time.  And the "other errors"
had been fixed in the past couple days.

Today I was going through the detailed logs to look for potential
problems.  The system is quite busy these days.  Some teacher had
called in on her own dime while an auto-dial was going on, and to my
surprise, I saw both calls apparently working correctly.
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