For every message you need 4 phones, 2 from you, and 2 for the other person. Prefferably those are not your phones, but rather public phones And you dont call / recieve from the same locations, prefferable different cities.

To transmit a message write out your text first Use a simple scheme like 1 - 3 - 5 - 7 - 9 or 1 2 - 3 4 - 5 6 - 7 8 /..etc Each individual text would harmless, (you might use a code book also for some words). Now these texts would only start to mean somethng if, you combined the words from the conversations So you replace words at 1 - 3- 7 - 7, with the words from the other conversation.

Such conversations are very hard to crack, the reasons why. - they dont know the scheme you where using - they dont know the codebook you where using. - it is simply unposible to check every phone call on earth and mix it with another call. the total sum of possible combinations = n * n-1 and thats just to much for any computer to check