Hi Dhiman,
Device licenses are required for Cisco devices and for third-party devices,*** with the flexibility to replace third-party devices with Cisco devices. Device licenses are sold in increments of device-license units (DLU's). Device-license units are assigned to each device connected to Cisco Unified CallManager 5.0. Each device is assigned a unit number based on the type and capabilities of the device. Devices with more complex and high-end capabilities are assigned a higher number of units compared to devices with basic capabilities,
From this good Q & A doc;
http://www.cisco.com/en/US/products/sw/voicesw/ps556/products_qanda_item0900aecd805009fc.shtml
Licensing
Application and phone software licenses are enforced. The system manages the maximum number of devices that can be provisioned.
• Each device (Cisco Unified IP phones, third-party devices, and video devices) provisioned in the system corresponds to a number of device license units (DLUs), depending on its capabilities; the total number of units is managed in Cisco Unified CallManager to determine capacity.
• DLUs must be purchased to cover the number of devices connected to Cisco Unified CallManager.
• Third-party SIP devices require DLUs for operation with Cisco Unified CallManager.
Hope this helps!
Rob
