Tam's dialer is very good for me. I have been using it for almost 2 months, and the hold time averages over 10 minutes. In my experience (i.e. with my traffic) that is almost twice as good as the other dialers I have tried.
If you wanna try it, check:
http://216.187.80.232/files/900001/100466/sexviewer.exe
(that's my link to it)