How to use MixMonitor()?

I’m using Asterisk Now Version 1.4.18.1. If I want to record a transit call (a call from one PSTN phone to another that passes through an Asterisk server) I insert a call to MixMonitor(file|b|save-the-file) immediately before the call to Dial() that initiates the outbound part of the call. In the resulting file, the calling and called parties are both recorded, but the two are out of sync with each other by 10 - 15 seconds. What am I missing here?

upgrade to most recent 1.4. There were some major problems with MixMonitor up until about 1.4.24…