VOIP over VPN, need suggestion please


Hi all,
I need suggestion about QoS over our network. I am using asterisk and SIP protocol.
We are curently hava connecition over VPN with 64 kbps. I am using codec g723.1 for our branch, and in head office using PCMU. But sometimes the voice is break up, when traffic is little bit high.
I request to our provider ask for QoS, using precedence 5 (6 and 7 for special user only).
Can someone give me suggestion to configure the QoS on my IP device. I am using ATA HT-488.
QoS layer 3 is 48 and QoS Layer 2 on 802.1Q/VLAN = 0 ,802.1p priority = 0.
Thank You in advance.

Best regards,