Unless you are dealing with a point-to-point circuit or MPLS connection between your branch offices, simply adding QoS to your SonicWall or other VPN/Firewall device won't be enough. If your SonicWall's are using public internet connections, they will have no control over the inbound data received for the WAN and outbound QoS will be stripped off once it hits your ISP's router.
In fact, added any QoS policies to inbound traffic can cause more network conjestion. Consider a SMB packet which has traversed your bandwidth limited download link and reaches the SonicWall. The SonicWall sees that this is Windows file sharing, which is low priority, and drops or delays the packet due to higher priority VoIP that is also being received. Well, the sending client gets the TCP retransmission and has to send that SMB packet again which increases the overall bandwidth usage on your link than if the packet had been allowed in the first place.
I guess my suggestion to increase VoIP quality is:
- Utilize MPLS to connect to your branch offices which will honor your QoS policies.
- When using the public Internet for VPN's, use links which are dedicated to VPN traffic only (so when Bob in accounting downloads 100MB of family photos from Flickr it won't affect VPN traffic).
- If you combine internet and VPN traffic on one connection do not set QoS on inbound/ingress traffic.