Convert Audio File of Unknown Format

In the early '90s a company called The Complete PC produced a line of internal PC cards that provided various combinations of modem, fax and voice mail under brand names like "The Complete Answering Machine" and "The Complete Communicator". We believe these cards were built on some sort of Rockwell chipset from the period. (If absolutely necessary we can research this further.) We have a number of voice messages that were recorded on The Complete Communicator but no system capable of playing them and we need a process or tool that can convert them into a modern audio file format. An example of one such message is attached -- an outgoing message (phone number long since changed) which can be made somewhat audible (but with a LOT of extraneous noise) by loading it as a 4kHz mono, 8-bit, unsigned, raw PCM file. It is unclear whether the noise is compressed data packets, command codes, or something else. The successful bidder will determine the format of the original data and provide a method to convert it directly, either by process or custom tool, into a standard audio format such as a Windows PCM wave audio file with clear fidelity, without relying on the original hardware (e.g., re-recording the audio through the original hardware is not an acceptable solution).