Download CCNA Collaboration Implementing Cisco Collaboration Devices (CICD).PracticeTest.210-060.2019-01-26.1e.197q.vcex

The DHCP response contains the phone IP address and the IP address of the TFTP server (which is usually a Cisco CallManager server). The response can also contain any of or all these common options:
IP address of the default router (gateway) 
IP address of the Domain Name System (DNS) server 
Domain name 
Reference: https://www.cisco.com/c/en/us/support/docs/voice-unified-communications/unified-ip-phone-7900-series/5710-phone-reg.html#topic4

Reference: https://en.wikipedia.org/wiki/Robbed-bit_signaling

Voice calls, either one-to-one or on a conference connection capability, require the following:
≤ 150 ms of one-way latency from mouth to ear (per the ITU G.114 standard) 
≤ 30 ms jitter 
≤ 1 percent packet loss 
17 to 106 kbps of guaranteed priority bandwidth per call (depending on the sampling rate, codec, and Layer 2 overhead) 
150 bps (plus Layer 2 overhead) per phone of guaranteed bandwidth for voice control traffic 
Reference: http://www.ciscopress.com/articles/article.asp?p=471096&seqNum=6

In general, QoS provides better (and more predictable) network service by providing the following features:
Supporting dedicated bandwidth 
Improving loss characteristics 
Avoiding and managing network congestion 
Shaping network traffic 
Setting traffic priorities across the network 
Reference: https://www.cisco.com/c/en/us/td/docs/ios/solutions_docs/qos_solutions/QoSVoIP/QoSVoIP.pdf