A System on Chip (SoC) is presented, which is well-suited for both conventional telephony via the Public Switched Telephone Network (PSTN) and Voice over Internet Protocol (VoIP). A highly flexible approach offers the possibilities to integrate several interface types, customizable by software download. Additional download capabilities facilitate the integration of voice-codecs for voice compression, algorithms for silent noise injection, far end line echo cancellation, and fax/modem termination. The modular design-style allows to reuse all major building blocks in future systems. The modular testconcept enables separated tests for all parts of the chip as well as configurable self tests for easy go/nogo-decisions.