What is the threat model? Even ROT-13 would thwart casual listening on or data harvesting. If you to be secure then you use voice over IPSec, PGPhone or any of dozens of other solutions. The idea that a commercial carrier can or should provide NSA-proof security boggles the mind. Nice masturbatory material though.
The paper above shows an easy birthday attack on such encryptions. Approximately 18% of 128 bit numbers can be expressed as a product of two 64-bit numbers. For such keys, if the ciphertext is C, consider all 2^64 values m1 and m2, and compare m1^e with C/m2^e. This can be done in about 2^64 time and memory, and if the plaintext is in that 18%, it will be found as m1*m2.
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