I am offering a $50 bounty to the person that submits code
that allows asterisk to detect whether a line has a dialtone,
is already in use, or is dead. Specifically, it must work with
my setup which is an adit 600 fxo connected to a TE110P
using a T1 line. $5 modems can do this, so it shouldn't be
very hard for asterisk to do this.

See bug report at:
http://bugs.digium.com/bug_view_page.php?bug_id=0002612

which mentions a way this might be done.

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NVLineDetect - Detects answer, busy, congestion, dialtone, dead air, talk, etc. on ZAP, SIP, IAX, and other channels. E-mail us for the code at info@newmantelecom.com.