However, a little further down the road is ubiquitous 5G. When that happens several VoIP vendors seem bent on simply connecting their service to 5G so workers will be able to access their business VoIP service wherever they are and across any device as long as it's 5G compatible. While some vendors do this with current mobile technology, the latency limits around 4G calling often make it a sub-optimal experience, not just for video and collaboration, but often simple voice traffic, too. 5G is the first service that promises the bandwidth necessary to make true mobile UC a reality.
Using an on-site PBX means you own the hardware of your phone system. You can modify the system to you exacting specifications, including changing phone numbers and adding extensions as necessary. You will also be in charge of ensuring the security of your data stores such as phone number listings and voicemail recordings.

Standard VoIP features like call routing, call forwarding, call recording, call transfer, call queueing, IVR, etc. See more standard VoIP features here
For a VoIP system to work, it needs a means of routing calls between users or to the outside world. In a cloud-based system, this gets handled by a virtual PBX. In the cloud, this routing is managed by your VoIP provider, which is part of why you're paying them. Whatever vendor is supplying that is also running a large PBX operation in a data center somewhere, and slicing off a little of it to dedicate to your organization in exchange for your subscription fees.