Historically, telecommunication companies have relied on what is commonly referred to as ‘circuit-switched technology’ to transport telephone calls. This technology establishes a 'permanent' connection between the calling and the receiving parties for the entire duration of the call, therefore keeping anyone else from using the physical lines while the call is in process.
The problem with circuit-switched technology is that it requires a significant amount of bandwidth dedicated to each call. Furthermore, circuit-switched technology can only support certain types of calls (i.e. telephone-to-telephone). Moreover, the hardware needed to run circuit-switched networks is very expensive because voice and data services must be carried by different wires, and therefore, need separate hardware to accommodate the two types of traffic. The high cost of this hardware has caused many traditional telephone companies to start using parts of the Internet infrastructure to connect PSTN calls. You may have already placed or received a call using VoIP technology without even knowing it!
Naturally, the traditional telephone companies pass along the costs of building and maintaining a circuit-switched network to the consumer in the form of higher rates for their telephone services. Telecom companies may save some money by borrowing from Internet bandwidth, but if your call is placed on a regular telephone using PSTN hardware, you won't see these savings.