MiSam,
Wow, I have never had that request before; congratulations that is a new one.
Uh, I have never tried this, obviously, but what if you set your phone to use the Alternate Transient Display and then set the timer for the Alternate Transient Display to a higher number? I know this will cause all of your transient displays to last longer though not just the Call Duration.
Thanks,
TE