This is one of the questions that are fumbled with by many an expert, and its not surprising since there is often an overlap of Telephone infrastructure and Telephony protocols, for instance, the ability to make phone calls over your computer as opposed to the cordless phone on your kitchen counter. In order to understand what are the different types of phone systems, we need to distinguish between protocols and infrastructure, the two distinctions between the types of phone systems that are available.
Another aspect to note is that transmission protocols are not systems, neither are proprietary handset designs; phone systems are those that demonstrate an operational architecture, a process flow that includes the sending and receiving real time voice transactions.
These are software or firmware based solutions that run on standardized infrastructure, or on common computing platforms, think of them like the instructions that control the voice communication over a certain system. Due to the unique application of some protocols, they are used exclusively as phone systems. Here are a few protocols that pass as phone systems, but purely because they perform the functions of a phone system without the need of specialized equipment:
1. VOIP (Voice Over Internet Protocol) - You'll probably argue that there are infrastructural setups that work exclusively with VOIP, true, but VOIP is a phone service that works because of the Internet, it is a set of instructions that treat an on-line transaction to send and receive a voice transaction over public and proprietary Internet services. It can be used over virtual or physical interfaces
2. ISDN (Integrated Services Digital Network) - This is a standard or protocol that utilizes data lines to simultaneously transmit voice, video and data. It qualifies as a telephone system since it is not limited to proprietary hardware constraints.
This refers to physical hardware that is installed for the purpose of completing a voice transaction from two distant locations. The infrastructure is an arrangement of voice management equipment, handsets and cabling. The following are the hardware based systems, regardless of the protocols they might support in their feature:
A twisted pair systems that uses pure voice control and switching technologies to operate a voice isolated network of 40 or more terminals, each with the ability to call to and for each other, and other telephone systems as well. It is somehow archaic, but it has not net lost it's value in innovation when using integrated systems.
Similar to PBX systems but allow users to select which lines are outbound, unlike PBX systems which automatically select the outgoing call allocations for the users. They are designed for larger voice networks.
This is one of the systems that are distinct through infrastructural system. It is a hybrid set up, modeled after PBX systems that run on an Ethernet network, using data standards to encrypt the voice communication along with data protocols.
This is a cheaper alternative to PBX systems, and normally work well with up to 40 users on the network. They are distinct by the fact that the firmware is contained in the handsets, and they are normally programmed by the users due to lack of universal support.
There are other legacy systems that are not mentioned, and they mostly have to do with private networks which need the anonymity with internal communication. However, many businesses and organizations are falling back on integrated solutions that give them the flexibility to work with the infrastructure they have already invested in is a great way to avoid problems with your phone system operations. When making your choice on which of the phone systems works for you, the function, investment structure and technology integration come into play as game changers.