OBi110 as FXO: incoming volume fluctuation on calls

Started by AMaliszewski, August 28, 2011, 01:18:52 PM

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I have two OBi110s configured as FXO trunks into an IP PBX and I can make and receive calls without much ado.

However, I'm experiencing a problem with near end incoming audio. Whenever the far end begins to talk, the volume is initially very low but quickly comes back to normal (around a second). This causes the beginning of far-end sentences to be cut off, which is annoying at best. Once they're talking audio quality is fine. The problem occurs only when the near end stops talking and the far end starts talking. There is no problem in the other direction. The problem occurs on every call, both incoming and outgoing.

To me it seems like some sort of overzealous echo cancellation, but I'm not totally sure.

Also, for what it's worth, I had a bit of trouble sending DTMF at first. To solve this increased the interdigit spacing and the problem went away. I don't know if the two are related.

I'd welcome any input. Thanks.


Thanks for reporting this issue.

We have been working on improving voice quality on calls through FXO port, and we like you to try our evaluation firmware for better call experience on the LINE port, please contact OBi support at, and state your 9-digit OBi number in your email.

Thank you,


Thanks for getting back to me so quickly.

Turns out the problem was the line impedance. It appears that choosing 900 ohms seems to have solved the problem with the clipping (and the DTMF can go out much faster).



Thanks for reporting back with your solution.

Impedance mismatch is usually the main cause to FXO voice quality issue, as what you experienced, and the line echo as well. The default LINE port impedance (600 oHm) is set for regular PSTN line in US, and it seems your PBX requires a different impedance setting in this case.

Hope this also help others with voice quality issue if any, thank you,