How to measure audio delay between two Asterisk channels?

I have the following Setup: A Centos machine with asterisk running that is connected to a Router via Ethernet. The Router answers the calls from my asterisk console and is itself again connected to my Centos machine via analogue RJ11 cable and a Digium PCI Card. (all for testing purposes of the router and running fine) So I call and pick up the call at the same machine.

How can I now measure the delay between the audio of both channels? I already tried to use the Monitor() function and evaluate delay signal based with cross correlation, but it seems like the Monitor function is not an accurate tool for recording at a precise time. (It doesn’t start recording when I start the Playback() of my testfile)

Is there another possibility to get the delay between caller and callee?