Voice quality is determined by the Differentiated Services Code Point (DSCP) value that you select (that is, 0-63). The default ...

Voice quality is determined by the Differentiated Services Code Point (DSCP) value that you select (that is, 0-63). The default DSCP value is 40 (101000). A network enabled for Differentiated Services (DiffServ) provides class-level prioritization based on Differentiated Services Code Point (DSCP) marking of the IP packets. Each DSCP value corresponds to a class of service for forwarding packets from the sender or intermediary router to the next router or receiver in the network.
The DiffServ RFCs recommend, but do not require, certain encodings.  For a description of the 6-bit DSCP field values in the Type of Service byte of any IP packet, see IETF RFC 2474.