When answering to a secure call, the recipient hears a sound similar to that of a fax. How can I fix this?

This happens due to operator-related issues. It is not possible to establish a GSM CSD data connection, in other words, the call is sent over the data line (safe), but is received over the standard voice line (unsafe). In this case you must ensure that the recipient has enabled the additional CSD data phone number, and that the caller has dialed the correct number for the new data line to accomplish a call in safe mode.