We have 2 t1's extended over fiber using rad's Optimux-4T1. connected is a predictive dialer we are getting a d channel error due to the voltage being to high i believe the voltage is 6.6db. Is there anything we can do to lower the voltage our provider says they can not do anything on their end and that it is checking out ok at the smarjack.

First of all, carrier is required to generate signal within a certain dB and dBx range. 6.6db is well within that, unless you have your card configured expecting something like 0dB.

Make sure you understand what the dB is and what it should be set at, then configure your card accordingly.