When it locks, or freezes, or whatever we want to call it... are you sure it is not the usual ATT first call not connecting thing? Sounds like the dialer never works until a reboot, just wanted to confirm that is correct.
The next time the dialer locks, go back to your home screen, hit MENU-APPLICATIONS-MANAGE APPLICATIONS-ALL and locate the dialer. Select it, and hit the FORCE STOP button. Try dialing out again. This is not a solution, but a potential work-around to save you time on a reboot.
Depending on your OS versions and status, there may be some recommendations to resolve permanently.