The interactive nature of telephony requires that TAPI be a real-time operating environment. Many of TAPI's functions are required to complete quickly and return their results to the application synchronously. Other functions (such as dialing) may not be able to complete as quickly and therefore operate asynchronously. Any given operation always completes either synchronously or asynchronously, and both types of operation are explained in the following topics. A list of all TAPI functions, which states whether each one completes synchronously or asynchronously, appears in TAPI Quick Function Reference.