Conventional telephone networks, whether public (PSTN) or private, bear several things in common. First, the phones used to make calls across them almost always use one- or two-pair physical connections. Second, the call-management device nearest the end user , be it a key system or a PBX, usually provides a dedicated, single-purpose circuit for each phone. The voice applications delivered by legacy systems are rigidly tied to the lower layers of the network. For instance, you can't get plain old telephone service from a cable company or a satellite provider because they can't provision copper telephone lines to your premises. Finally, the capacity of the data links used to carry traditional telephone calls rarely increases over time. It remains fixed, forever tied to the quantity of cable pathways between one point and the next .
These traits are common among legacy voice setups, whether they consist of heavy-duty TDM-bus PBX systems or just a few analog phones connected to the PSTN. Incidentally, VoIP doesn't exhibit these traits, but since your transition to VoIP may be incremental, it's important to understand the "goods and bads" of traditional circuit-switched telephony.