Hi Mostafa,
You will probably get lots of answers to this good question. The explicit Voice vlan is certainly a Cisco "best practice" here is a clip from the SRND;
When you deploy voice, Cisco recommends that you enable two VLANs at the access layer: a native VLAN for data traffic and a voice VLAN under Cisco IOS or Auxiliary VLAN under CatOS for voice traffic.
Separate voice and data VLANs are recommended for the following reasons:
Address space conservation and voice device protection from external networks
Private addressing of phones on the voice or auxiliary VLAN ensures address conservation and ensures that phones are not accessible directly via public networks. PCs and servers are typically addressed with publicly routed subnet addresses; however, voice endpoints should be addressed using RFC 1918 private subnet addresses.
QoS trust boundary extension to voice devices
QoS trust boundaries can be extended to voice devices without extending these trust boundaries and, in turn, QoS features to PCs and other data devices.
Protection from malicious network attacks
VLAN access control, 802.1Q, and 802.1p tagging can provide protection for voice devices from malicious internal and external network attacks such as worms, denial of service (DoS) attacks, and attempts by data devices to gain access to priority queues via packet tagging.
Ease of management and configuration
Separate VLANs for voice and data devices at the access layer provide ease of management and simplified QoS configuration.
To provide high-quality voice and to take advantage of the full voice feature set, access layer switches should provide support for:
802.1Q trunking and 802.1p for proper treatment of Layer 2 CoS packet marking on ports with phones connected
Multiple egress queues to provide priority queuing of RTP voice packet streams
The ability to classify or reclassify traffic and establish a network trust boundary
Inline power capability (Although inline power capability is not mandatory, it is highly recommended for the access layer switches.)
Layer 3 awareness and the ability to implement QoS access control lists (These features are required if you are using certain IP telephony endpoints, such as a PC running a softphone application, that cannot benefit from an extended trust boundary.)
Spanning Tree Protocol (STP)
To minimize convergence times and maximize fault tolerance at Layer 2, enable the following STP features:
PortFast
Enable PortFast on all access ports. The phones, PCs, or servers connected to these ports do not forward bridge protocol data units (BPDUs) that could affect STP operation. PortFast ensures that the phone or PC, when connected to the port, is able to begin receiving and transmitting traffic immediately without having to wait for STP to converge.
From this CCM SRND doc;
Cisco Unified Communications SRND Based on Cisco Unified CallManager 4.x
Network Infrastructure
http://www.cisco.com/en/US/products/sw/voicesw/ps556/products_implementation_design_guide_chapter09186a00806e8c42.html
Understanding Voice VLAN
From this doc;
http://www.cisco.com/univercd/cc/td/doc/product/lan/cat3560/12220se/3560scg/swvoip.htm#wp1033848
Hope this helps!
Rob
