Abstract: Full-duplex, concurrent voice and non-voice communication over the public switched telephone network (PSTN) is maintained by a communication interface apparatus. A voice only connection is established between two sites initially. Concurrent voice/non-voice connection then is established by pressing an engage button on the communication interface apparatus at each site. Voice communication is temporarily lost, while the connection changes from a phone-to-phone voice-only link to an interface-to-interface voice/non-voice data link. The communication interface apparatus includes a digital signal processor for converting and compressing digital voice data into a voice data packet, a modem for maintaining communication over the PSTN and a micro-controller for managing the establishment and maintenance of concurrent voice and non-voice data communication.