Because they're working across such a broad array of features and channels, many of today's phone systems are adopting the moniker of Unified Communications-as-a-Service (UCaaS). These are generally cloud-based, virtual PBXes (private branch exchanges) that include at least one (usually multiple) software clients to enhance their functionality on the web, desktop, and especially mobile devices. Even residential VoIP systems come with features that are simply impossible with a conventional telephone system.
For one, be sure to understand QoS (mentioned above). This means going beyond understanding the concept and moving to how the networking equipment in your office -- or your employees' home offices if you're still at that stage -- can actually implement QoS. Most business-grade networking hardware will be able to handle QoS in more than one way, so testing which method will handle voice traffic more smoothly in your environment is important.

Phonewire provides business telephone system sales, nationwide installation, text messaging, and technician services. One of the services they offer is voicemail transcription, ‘typed by actual human beings’. Transcriptions are sent to an email address with the audio file attached, and also include a Gender ID with the Caller ID information to tell you whether the caller is male or female. Phonewire also offers a developer API that integrates with most business phone systems and voicemail services, including OnSIP.
In order to determine the number of users you need for your VoIPstudio phone system, count the total number of devices you need, including desk phones, softphones, conference phones, and fax machines.