A voice changer is not really necessary when it comes to phone monitoring done by law enforcement agencies: they will know exactly your identity, location, calls and messages path, the voice content of your calls and text content of your messages, etc. A voice changer will give you a false sense of security, unless you intend to use it for a prank. Also, in the past 3 years interception systems that use voice recognition feature for automatic target detection and call recording is not used as primary filtering tool, due to the fact that sample voice (needed for voice recognition) sounds different from phone to phone, due to different hardware capabilities. Hence, false positives emerging with an unwanted high rate. Read more here.