"My question is, will share class OUT-VOICE its free bandwidth if OUT-DATA class is fully utilized?"
I believe it should.
"I have issues, data class is dropping too many packet even the link is not fully utilized."
Often that's a symptom of bursty traffic, i.e. the available bandwidth is overrun during a microburst, drops happens, TCP goes into CA. (See classic TCP saw tooth bandwidth graphs.)
Increasing your class OUT-DATA buffers might reduce drops.