Routing in the PSTN is the process of forwarding telephone calls between the constituent telephone networks that comprise the public switched telephone network (PSTN).

Telephone calls are routed across a network of potentially many switching systems, often owned by different telephone carriers. Switching systems are connected with trunks. Each switch may have many neighbors. Neighboring switches owned by different operators are connected at interconnect points.

The PSTN is a network that uses destination routing to direct calls from origin to the recipient. It is not a full mesh network with the nodes of every operator directly connected to those of every other, which would be impractical and inefficient. Therefore, calls may be routed through intermediate operator networks before they reach their final destination. Efficient least-cost routing is an important procedure in PSTN routing.

Each time a call is placed for routing, the destination number (also known as the called party) is entered by the calling party into their terminal. The destination number generally has two parts, an area code which generally identifies the geographical location of the destination telephone, and a telephone number unique within that area code that determines the specific destination terminal. The telephone number may be subdivided into a prefix that may identify a more specific geographic location or a telephone exchange, and the rest of the number. For example, in the number (301) 555-1212, 301 is the area code and 555 is the prefix. (The area code is sometimes known as an "NPA," and the area-code and prefix combination is known as an "NPA-NXX.") In the United States, prior to 2021, in some locations, if the call was between two terminals in the same local area (or, two terminals on the same telephone exchange), then the area code could be omitted. With the introduction in October, 2021 of the national suicide hotline number 988, in most cases, the ability to dial local numbers without the area code was eliminated, and the area code is required for a call to a regular phone number to complete.

When a call is received by an exchange, there are two treatments that may be applied:

Each exchange in the chain uses pre-computed routing tables to determine which connected exchange the onward call should be routed to. There may be several alternative routes to any given destination, and the exchange can select dynamically between these in the event of link failure or congestion.

The routing tables are generated centrally based on the known topology of the network, the numbering plan, and analysis of traffic data. These are then downloaded to each exchange in the telephone operator's network. Because of the hierarchical nature of the numbering plan, and its geographical basis, most calls between telephone numbers on the same network can be routed based on their area code and prefix using these routing tables.

Some calls, however, cannot be routed on the basis of prefix alone, for example non-geographic numbers such as toll-free or freephone calling. In these cases the Intelligent Network is used to route the call instead of using the pre-computed routing tables.