In SX Microcontrollers, SX/B Compiler and SX-Key Tool, George Herzog wrote:

We just got a new SX/B [that is Basic like PBasic] and it is better than ever.
But I am afraid all the modem and telephony work has been done in Assembler [circa 1999] and Virtual Peripherals.

I did locate a schematic, but it looks to me that one might easily buy a Caller ID modem for about the same cost as building the device.  And you would have a 'telephone company approved' interface.

There are basically 5 parts to the construction.  The ring indication circuit, the impedance matching ring and tip isolation for incoming data, an OPamp filtering circuit, the SX as decoder/converter, and the RS-232 chip [a Max232] for output.

Still, someone mentioned this was for their employer.  So buying the modem is a conservative business-like approach.  You have warrenties, a package in a chassis, and so forth.

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