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FXO Line quality issues, voltage problem?

I've been having an issue with a UC320 with FXO lines from Frontier in northern california. Complaints from the customer are low incoming audio, low outgoing audio, echo, and general poor audio quality. I have tested and set impedence on all lines, toyed with FXO port gain, etc. I have an open case with small business support, and they have gone through it as well.

As a test, i moved some of the lines around, and swapped 'line1' and 'line3' so now line 1 is in port3 and vise versa. This seemed to help after i made the changes when we got an incoming call over the main line, which is now in line 3.

The support engineer i was working with said that in his experience with the UC320, ports 3 and 4 seem to do better with line voltage issues than ports 1 and 2. He could not find me any references as to why, and also couldn't find what the exact line voltage tolerances are for the ports.

Can anyone out there assist with this? Is there any truth to the fact that ports 1 and 2 may act differently than 3 and 4? I have a buttset that will tell me line voltage, what numbers should i be looking for, and what range is acceptable for the UC320?

Is there anything else i can try to resolve these line quality issues? i've already got an RMA unit on the way, incase i just have bad ports.



Christopher Edgeworth
Frequent Contributor

Hi Craig,

What is your SBSC case number?  (PM if you don't want it in public).  The voltages are based upon the telco spec for the region/country.

All of the ports are the same.  In a PBX dialplan the FXO are selected from the lowest port number to highest.  If you have a problem on say a FXO in port 3, you may not see it as often because the FXOs in port 1 and 2 have to be in use before we hit port 3 on outbound calls.  Inbound calls will depend on how the rotary group with your FXO provider is provisioned.