D-Link IP Telephony System DVX-8000/M/E

DVX-8000 is the right choice. Especially for normal office setup (for call centers see DVX-9000  and higher), 

The DVX-8000 supports an enhanced switchboard with unlimited buttons, instant messaging and voice mail explorer.

DVX-8000 phone system series supports up to 300 users and up to 60 concurrent calls in normal office settings. In hybrid settings, it can support up to 60 analog extensions which make it perfect for implementations with the high-density existing infrastructure that is costly to replace, such as in some hotels, hospitals and other organizations. DVX-8000 also supports up to 2 PRI lines. Running EasyVoIZ operating system.