In 1989 manufacturers started to define an application interface which would be accepted in the growing ISDN market. To get an acceptable result, the focus of this standard was the possibility of running the national ISDN protocol, for an ETSI ISDN protocol standard was not available at this time. Work on this application interface was finished in 1990 by a CAPI working group consisting of application providers, ISDN equipment manufacturers, large customers / user groups and DBP Telekom.

COMMON-ISDN-API Version 1.1 was a great step towards opening the national ISDN market in Germany. Meanwhile almost every German ISDN solution as well as an increasing count of international ones is based on COMMON-ISDN-API; there exists a well accepted conformance test laboratory at DBP Telekom.