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Beginner

POTS Line High Voltage

I have a client site that is experiencing some randomly weird issues with pots lines and fxo cards on a UC560.  At random times, the phones in the office ring when a call comes in but the users can't answer the call and the phones keep ringing when they try to answer.  I opened a support case and did some troubleshooting.  I was told to have the telco come out and test the voltage on the lines.  They did and reported back that they were a little high, coming in at -50-51v depending on the line (8 lines total).  We also tested at another site for the same client and all 12 lines there came back at -48v.

I've done some searching and haven't found whether this is breaking some sort of threshold or if this would cause a problem.  They mentioned that there may be some sort of suppressor that we could place on the lines to keep them at -48v.  Does this sound viable or is there something else I should be checking?  On these same lines, I also have an issue with the ports not going to onhook automatically until the call has been disconnected for several minutes.  This is only happening at this site.  I have tried some supervisory disconnect settings to no avail.  Any info is greatly appreciated!

Thanks.

Travis

Everyone's tags (7)
12 REPLIES 12
Beginner

Re: POTS Line High Voltage

Check out mike sandman website, he has loop current regulators that may help...

you realy need the phone company to make sure there lines are correct, they are charging your customer for a service and not providing it

the cpc disconnect is so important for having the auto attendant work properly

Beginner

Re: POTS Line High Voltage

Thanks.  I'll see if I can pressure the telco a little more.  Does the voltage issue sound viable?

Travis

Participant

Re: POTS Line High Voltage

I have seen the behavior you described a couple times in the past. One situation turned out to be a hardware failure with the UC520, the other turned out to be an issue with the cross connect. I changed the ends from 4 pin RJ11 to 2 pin and that cleared up the issue.

Cole

Beginner

Re: POTS Line High Voltage

Unfortunately I can't change the pins since I'm already on 2 pin rj11.  What issues were you seeing that were caused by the hardware failure?  Do you know specifically what part of hardware failed?  Did you end up having to replace the whole unit?

Thanks.

Participant

Re: POTS Line High Voltage

I was seeing a handful of issues with the defective unit.  The biggest problem was IP phones randomly rebooting, but there was also the same behavior with inbound calls not being able to be answered. Not sure what specific part failed but I had to get the whole UC520 replaced before the problems went away.

Cole

Beginner

Re: POTS Line High Voltage

Does anyone have any additional experience on the fxo ports not disconnecting immediately when a call is ended?  It seems to be a problem mostly on the calls that end up in a voicemail box and the caller hangs up before leaving a message (or after leaving a message).  If I call and get a voicemail greeting and I hang up, the greeting finishes out and the handler even records a blank message for a couple of minutes before the call is disconnected.  If I call and get a voicemail greeting and wait for it to finish, then leave a message and hang up, it will properly record my message and then record a couple of minutes of dead air after the message.  Eventually the call is disconnected and the fxo port goes back to onhook status.

I've tried so many settings to detect the hang up and I've contacted the telco a few time.  They originally said that they were not sending a disconnect tone so I asked if they could and they responded and said now all I need to do is configure supervision on my ports (which was already done) but I'm still seeing the exact same behavior.

Beginner

Re: POTS Line High Voltage

what your looking for from the carrier is a 450ms open when the caller hangs up, one easy way to test this is to go to site with a simple voltage meter put one lead on tip one lead on ring call the line from your cell phone answer the phone then hang up with your cell phone if you dont see the meter go to 0 for 450ms than they are not sending cpc

Enthusiast

Re: POTS Line High Voltage

Can you post a running config of your just your FXO Voice ports?

Beginner

Re: POTS Line High Voltage

Sure, here's port 0/1/0 which is the port I have been doing most of my testing on.  All ports are configured the same with the exception of the destination.  I have tried several different supervisory disconnect setups but nothing has worked thus far.

voice-port 0/1/0

trunk-group Line23 64

supervisory disconnect anytone

timeouts call-disconnect 4

timeouts wait-release 4

connection plar opx immediate 6285

description Configured by CCA 4 FXO-0/1/0-Custom-BG

caller-id enable

Enthusiast

Re: POTS Line High Voltage

Look like you have standard config.  Try this config to test it out.  You can just copy n paste it.  Make sure you call the correct number that corresponds with the FXO port. If this doesn't work than u can rule out issues with the config.  An FXO port with the connection PLAR command should work right out of the box.

Test Config

voice-port 0/1/0

no trunk-group Line23 64

no supervisory disconnect anytone

no timeouts call-disconnect 4

no timeouts wait-release 4

no connection plar opx immediate 6285

no description Configured by CCA 4 FXO-0/1/0-Custom-BG

connection plar opx 6285

caller-id enable

Old Config

voice-port 0/1/0

trunk-group Line23 64

supervisory disconnect anytone

timeouts call-disconnect 4

timeouts wait-release 4

connection plar opx immediate 6285

description Configured by CCA 4 FXO-0/1/0-Custom-BG

caller-id enable

Beginner

Re: POTS Line High Voltage

Okay, nothing has worked.  I've asked Cisco AND CenturyLink and no one seems to be able to tell me if 50-51v is out of the acceptable range for voltage on a POTS line.  Does anyone know what range is acceptable for a POTS line into a UC560?  My client is continuing to have random issues with incoming calls and they are pressuring me for a definitive answer on a range or threshold for voltage.  Anyone know anything?

Beginner

Re: POTS Line High Voltage

The POTS phone line, with all phones on-hook, should measure around 48

volts DC. Taking a phone off-hook creates a DC signal path across

the

pair, which is detected as loop current back at the central

office. This

drops the voltage measured at the phone down to about 3 to 9

volts. An

off-hook telephone typically draws about 15 to 20 milliamps of DC

current

to operate, at a DC resistance around 180 ohms. The remaining

voltage

drop occurs over the copper wire path and over the telephone

company circuits.

These circuits provide from 200 to 400 ohms of series resistance

to protect

from short circuits and decouple the audio signals.

On 1/3/11 8:51 AM, "thawkins@triggerfishadvisory.com"