An on-premise PBX (Private Branch Exchange) is a business phone system that is physically located on the premises of the organization using it. It consists of hardware, software, and phone lines that are owned and managed by the organization itself. The PBX is responsible for managing and routing incoming and outgoing calls within the organization’s phone network. 

A cloud PBX, on the other hand, is a phone system that is hosted and maintained off-site by a third-party provider, usually in data centers. The phone system is accessed and managed through the internet, and the organization does not need to own or maintain any physical PBX hardware or phone lines. The cloud PBX provider is responsible for managing and maintaining the phone system, including upgrades, backups, and security. 

One key difference between an on-premise PBX and a cloud PBX is how phone lines are handled. With an on-premise PBX, separate phone lines are typically required for each concurrent call that the organization wants to make or receive. These lines may need to be provisioned and managed separately, and the organization may have to work with a telecom provider to set up and maintain them. 

In contrast, a cloud PBX typically uses virtual phone lines, also known as VoIP (Voice over Internet Protocol) lines. These virtual lines are provisioned and managed through the internet and come bundled with you cloud PBX system.