I have about 30 embedded modems based on the Conexant RC56D/SP. These units have a TTL-level serial interface and run off +5V. The interface is a 34-way 2x17 100mil header, for which I do have pinouts.
RJ11 jacks are not fitted, although there is space for them on the PCB. The phone line signals are run to the 34-pin host connector, and the particular unit that used these modems had the RJ11s somewhere else on the device. All line isolation components are present on the modem board, you can just solder on a jack and they'll work.
No FCC stickers on these modems (they are samples; maybe one or two have FCC stickers on them), so they are technically unapproved, though I know the modem itself has at least FCC. Designed for US call progress cadences.
Free to good home, you pay postage from ZIP code 11375. Yes, I will send them internationally but you have to cover any duties, taxes, bribes etc. First email gets them all.