Analog PBX integration

I had not ever had to interface to an Asterisk PBX analog, and perhaps ,i assumed that there were no problems.

I ask if you can kindly enlighten me and let me solve the bad question.

I have a server with one asterisk tdm410p with 4 FXO, but currently, I use only one.At the FXO port arrives
internal pbx.I use asterisk now.when call me from inside the PBX with an analog phone, chimeras attached to the inside of the PBX FXO, I rang my phone sip.Contrary will still work.

The problem is that the SIP phone feel, without exception, every word he utters.
I used fxotune seems to me removed the echo, but only on calls that do not pass the analog PBX.

Someone has no idea why and how to solve the mystery?

Thanks, I hope in your answer.