Pinouts
Contact assignments, or pinouts, vary by application. Telephone network connections are standardized by registered jack numbers, and Ethernet over twisted pair is specified by the TIA/EIA-568-B standard. Other applications have no standardization; for example, there are multiple conventions for use of 8P8C connectors for RS-232.
For this reason, D-sub-to-modular adapters are typically shipped with the D-sub contacts (pins or sockets) terminated but not inserted into the connector body, so that the D-sub-to-modular contact pairing can—and must—be performed by the end-user.
Read more about this topic: Modular Connector