DADHI Limitation on POTS lines

I started out using Asterisk about a year ago and had everything going swimmingly using the ZAPTEL drivers. About 6 months ago we decided that we needed to be on the L&G 1.4 or 1.6 release, so now I’m running 1.4 SVN with the DAHDI drivers. This all works pretty good except that ZAPTEL seems/seemed to be able to control the line much better than DAHDI does. If I did a call file using Zaptel to call a number and play a message, Zaptel would wait for the connection then play the file. DAHDI begins to play the file as soon as dialing is completed. I’m told that POTS does not supply enough bits for DAHDI to do line control, but why did ZAPTEL seem to be able to handle this?