When buying a phone system - make sure what you think you're getting is really what is in the contract:

CallerID - Make sure it includes the CNAME info and not just the number

Individual Voice Mail  - Make sure the system just doesn't have a global voice mail

Music On Hold 

IVR - Complex attendants and menus, including a directory.

After Hours System - does the system allowth you to change the logic based on the time.

Remote Voice Mail Retrieval - Can you call in and retrieve your voice mail.

Voice Mail To Email - Will the system email you your voice-mails.

Echo Canceling - hardware based. Software based systems are not good enough.

Self Management - do you have the software and manuals to do typical tasks yourself.

Intercom - can you intercom someone without them having to pick up the phone.

...

A lot of phone system vendors will imply that these things come with the system, but want to charge you later when you find out that they're missing.

All modern systems need to have a hardware based echo canceling system.