How to use a Windows 10 computer to make and receive POTS/PSTN phone calls in 2019?

Question

I would like to connect a PSTN/POTS, RJ11 (or RJ12) cable to a device and connect that device to a Windows 10 USB receptacle.

I would like to be able to make and receive phone calls using the default microphone and speaker that are connected to the Windows 10 computer. I am not interested in an additional, external sound card or audio devices (mic and speakers) for phone conversations.

We plan to include this device inside of a larger, consumer product, so smaller form factors are preferred (e.g., USRobotics 5637).

To be clear, even though VOIP/SIP solutions are great, they are not an option in this case, please do not recommend them here, I am only able to connect RJ11, PSTN devices.

I have tried a $10, no-name USB Voice modem from Amazon, Hiro v92 voice/fax modem, the USRobotics 5637 and the Way2Call Hi-Phone LITE.

The no-name, Hiro and USR modems all fail because they are "half-duplex." This means they will allow recording and playback of .wav files, but not real time bi-directional voice streams.

The Way2Call appears to be working with full-duplex audio streams over the USB connection, but this solution has a handful of tradeoffs that I'd like to avoid.

Are there are any alternatives available to us?

Any pointers, tips or help would be greatly appreciated!