Landline modems are by far the most secure. But even those modems call out to fiber optic cables that run on the same switches as the internet and cellular networks. ISDN modems a communication protocol that uses something like block chain technology is the best we can offer. ISDN give use speed (less chance to get intercepted) and data validation that audio modems just don't.

Those option aside, physically shipping the units in armored trucks with tamper tapes installed and physical tapes as a backup is the best option.