Hi Dev,
I am assuming that we are talking with respect to Communications Manager version 5.X and Later.
With this release Device License Unit concept was added which enforces phone license.
Now every phone consumes certain DLU (Device License Unit) which varies depending on the phone model.
For example 7940 will consume - 4 DLUs
7911 - 3 DLUs
So you purchase DLUs typically available in a pack of 10 whcih needs to be uploaded on teh CUCM. So lets say you have got 100 DLUs loaded on the CUCM. YOu add 5 numbers of 7940s. Eeach consumes 4 DLUs hence these 5 IP Phones will consume 20 DLUs.
On the CUCM as soon as these phones register from the pool of 100 DLUs 20 will be consumed and you'll be left with 80 DLUs which will be consumed by other devices.
For your reference I am attaching an excel sheet which gives you the number of DLUs required for various IP Phones models.
-> Sushil
