Question

How can we make digital secure module for land telephone line???

Answer 1

Very good question
I'd love to know.
At the high-level design stage we need to do this:
Analogue-to-Digital converter to Modulate (and Demodulate) the signal
Then process with a cryptographically secure module
Arrange a secure method to share the encryption / decryption keys
Try it out.
Are you thinking on your regular phone (PSTN) ?
Are you thinking using a public key system (asymmetric) or a secret key system (symmetric)?
cheers