VoIP Ready: The world is moving away from legacy PSTN lines and towards VoIP. Make sure your PBX can support IP stations (phones) and IP trunks (service). SIP is the current de facto standard, so don’t buy a phone system that doesn’t support it.
Voice Messaging: Once upon a time, voicemail was an optional add-on. Today, it’s table stakes. Look for PBXs that can forward voicemail messages to your email as attachments. If possible, look for IP phones that support visual voicemail.
Mobility: Most businesses have at least some road warriors who spend much of their time out of the office. Make sure your PBX supports mobility features like Find Me / Follow Me, remote IP extensions and fixed / mobile convergence.
Conferencing: One of the best ways to cut down on travel costs is teleconferencing. Make sure your phone system has native support for true multi-party conferences (not just basic three-way calling).
Reporting: If you can’t measure it, you can’t manage it. Make sure that the PBX you pick includes basic call history reporting features.
If internet goes down you are still able to call local extensions.
And much more...