As a phone system retailer and wholesaler, I am often asked what the cutoff line is for the total number of lines one needs before they should move out of a key system and into a PBX. For the uninitiated, a key system is based off of internal terminal technology, meaning the line routing equipment is contained within the actual phones, with one phone or small box serving as the main routing point. A PBX, or Private Branch Exchange, has a central terminal that connect telephone company lines directly to the lines and equipment in your business. Key Systems can only handle so many lines, while PBX systems are almost unlimited in their capacity.

So, just how many lines can a key systems handle? The short answer is roughly forty to fifty, depending on the manufacturer of the equipment. The real answer to that question is a little more complicated and has a lot more to do with your business and less with a specific number.

There was a time when hard and fast rules applied to the capabilities of key systems. Until just a few years ago, relatively few phone system manufacturers were placing advanced features on key systems and reserved them exclusively for larger PBX. Additionally, most key systems had a maximum handset capacity of forty, and rarely would any key system accommodate more.