How many trunks to the PSTN will you need? This will dictate the number of simultaneous incoming and outgoing calls you will be able to support. If you have an existing PBX, you should be able to do a "busy study" on trunk utilization to obtain the number of concurrent, simultaneous calls supported. You can also ask the phone company to do this for you, but don't expect it to give you results overnight.
Next you need to determine how you want to handle incoming calls. Most vendors support DID (direct inward dialing), which means that every phone can be reached from the outside. But if you want inbound calls fielded by an operator, make sure the system can be set up with an attendant console. Additionally, be sure you can monitor how quickly calls are answered. Another option is an automated attendant to direct calls to specific extensions through a menu system. Automation is fine, but be sure that the system can supply you with reports showing the call-completion rate. If a lot of callers are bailing out before they are satisfied, you'll want to make adjustments.
1. Make sure the PBX supports your processes and can meet your end-user requirements.
2. Determine how much incoming and outgoing call activity your organization gets and how to handle incoming calls.
3. Verify that your data network can support VoIP.
Level Playing Field