Purchasing the correct number and types of user licenses can be complex. I didn't appreciate the intricacies until I was near the end of reviewing the proposals.
Licensing can vary with every system. I looked closest at the Avaya and Cisco licenses and came across a number of issues. Purchasing the wrong mix initially can cost you more in the long run.
What does each license cover? I needed a sales rep to walk me through Avaya's licensing options. The licenses you need by depend on your staff being mobile workers, office only workers, and/or remote workers. Do they need a phone or just a license and software? Do they need voice mail? I suggest making a matrix of features and licenses.
The licensing structure might change every few months so double-check that your information is current. For example, the Cisco license changed after the proposals were sent to us. The new license cutting our costs by $2,500 by eliminating the need to purchase separate licenses for mobile Jabber.
Can a license be traded in? I almost write "upgraded" but in some cases one license might be different from another but not necessarily better. In the case of Avaya and Cisco you cannot exchange or upgrade licenses. If you bought the wrong license you need to buy another one of the correct type. This can make planning painful since you don't want to overbuy the more expensive licenses, but if you plan the wrong mix you may end up buying more licenses later on even though your staff size has remained constant. I would like the PBX companies make it possible to upgrade or exchange licenses after purchase. A flatter licensing structure would also help.
The most important difference is between licenses for phone assigned to people and for public phones. A public phone license is usually used for conference rooms and guest phones. They do not include voice mail or any advanced features.
Also think about any voice mailboxes that are not associated with any phone. We bought 3 voice mail boxes for department use.