Encryption method on a POTS modem?

I’ve got a business need to have a set of computers accessible via modems through a POTS setup. I’ve also got a mandate to encrypt connections. (I realize that the likelihood of someone tapping a POTS line is rather low, but we need to encrypt anyway.) Most of the solutions seem to center around having a hardware-encryption-capable modem on either end of the connection, and let them take care of the traffic. Does anyone know of a method using a single piece of (server-side) hardware, or some encrypted modem protocol? Thanks!

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