asterisk users - Apr 2006 - Dial(L(x...)) distinct to SET TIMEOUT(absolute)

I have been experimenting with setting call time limits, and modifying
those limits on the fly. My application is a case where a user has a
time limit on their call, determined by the user's credit. The time
limit may be extended by the provision of more credit.

It is desirable to use the L flag of the Dial() application, as it gives
the user audible feedback.
I was hoping to be able to modify this limit with the AGI command SET
AUTOHANGUP, or the inbuilt SET(TIMEOUT(absolute)) function.
I have found that with both of these methods, I can /shorten/ a call.
However, if I specify a value greater than the duration given to
Dial(,,,L()), the Dial() duration still applies.

It would appear as though the Dial() and the AUTOHANGUP (or
TIMEOUT(absolute)) methods are using distinct counters. This seems
unfortunate from my perspective!

Does anyone have any insight on this?
I am new to this list; perhaps it is not the appropriate list for such
discussion?

cheers,
Mike