A PBX system allows you to set extensions, customer management systems, auto-attendant, call forwarding and more to route calls within your organization. This operation is handled by the server private branch exchange (PBX). Virtual PBX service will be set up, maintain, and host your exchange server off-site and you just buy an IP phone and an internet connection interact with the web-based administration panel.
Self-Hosted PBX bringing in-house exchange server (on-premise). Benefits and trade-offs vary, but in general, it is a virtual PBX service for small businesses, and Self-Hosted PBX is a system for medium and large businesses with heavy call volume. You can also get your business hosted PBX via https://www.smartcombusiness.com/hosted-cloud-pbx/
You can use an analog phone line or digital VoIP PBX with the Self-Host. Virtual PBX services are limited to VoIP. It just focuses on cost savings (typically 50%) associated with business-class VoIP communications.
The general consensus is that the Virtual PBX services are ideal for small businesses with 1-10 employees only. With Virtual PBX, you share exchange 3rd party server with others, thus, sharing hardware and maintenance costs.
You must pin point your needs and carefully research your preferred virtual PBX provider. Costs are generally between $30-120/month for complete packages (minutes included). Extensions, may or may not be free depending on the provider. Your marginal cost will increase compared to self-hosted PBX systems as you add more users.