The configuration of QoS in the 3850 line has been improved due to its implementation of MQC (universal QoS configuration model) configuration instead of the old MLS QoS (platform-dependent QoS configuration) commands from the 3750 and 3560 lines of switches.
On the 3650/3850 series, QoS is enabled by default and all packets by default are trusted (the L2/L3 QoS marking is preserved), unless you change it with an application of a specific policy map on the ingress or egress interface.
There are two queues configured by default on each wired interface on the switch. All control traffic traverses and is processed through queue 0. All other traffic traverses and is processed through queue 1.
All routing control traffic in the network uses IP precedence value 6 by default. IP precedence value 7 also is reserved for network control traffic. Therefore, the use of IP precedence values 6 and 7 is not recommended for user traffic.
For wired or wireless ports that are connected to the switch (end points such as IP phones, laptops, cameras, telepresence units, or other devices), their DSCP, precedence, or CoS values coming in from these end points are trusted by the switch and therefore are retained in the absence of any explicit policy configuration.
So all in all, IP phones etc. are trusted and the DSCP/Precedence/CoS values retained, when nothing else has been configured.