While most commercial phone systems offer standardized features such as voice mail, call forwarding, transferring and hold music, not all are created equal. With three different options for businesses, it's important to understand that the phone system you choose should be reflection of your businesses size and where you expect to be several years down the road. If your company has a very small number of employees, a KSU-less system might be the right choice. KSU-less systems offer a high degree of functionality for a much lower initial investment. KSU-less phones are not tied together in the sense that traditional phone systems are. Instead, the actual handsets provide commercial phone system features, instead of connecting to a central control unit. 

For mid-sized companies, key systems offer more features and are expandable for normally up to 35 handsets. Key systems utilize a central control device called the key system unit (KSU) to manage the connected network of phones within your business as well as any outgoing or incoming calls. In recent years, key systems have become so advanced that many rival PBX systems in terms of standard features and customization.

For large companies with over 35 to 40 employees, a PBX system is the way to go. PBX systems offer the greatest amounts of functionality are highly expandable. A PBX central terminal controls all phone systems in the same manner that a key system does, but can do so for a substantially greater number of phones. Almost all PBX systems come with the standard features necessary for phone system operations as they pertain to large businesses. PBX systems are highly programmable and can be configured to meet the individual needs of any business. The only downside to PBX systems is the overall expense compared to key systems and KSU-less systems. Still, if your business needs over 35 handsets or may in the future, PBX is your only choice.

There factors that will determine the size of the system you need are the number of lines and the number of extensions you will need. Lines are the number of outside phone lines your phone system can simultaneously connect to. Extensions are needed for every handset or other device that the company needs to connect to the phone system. Knowing both can give you an idea of which phone system is right for your business.