A telephony application should be able to transmit and receive voice band data. To transport voice over an ISDN line, the adapter must support the Bearer Capability for speech for 3.1 KHz audio or for 7 KHz audio. Support for speech and 3.1 KHz audio means that the adapter is capable of modulating the analogue voice signal into a digital data stream. There are two PCM flavors. North American and Japanese equipment use the mu-Law Algorithm.
European equipment uses the A-Law Algorithm.
Eicon ISDN adapters also support 7KHz audio which uses the adaptive PCM algorithm to compress digital voice signal. An application can control the voice connection by using Eicon CAPI 2.0 Software Development Kit. After the voice connection has been established, the application has access to the voice data stream. The application can play the received voice signal using a third party sound card. Through the Eicon CAPI 2.0 Software Development Kit the application can also access the calling party number as well as the redirecting number.