Strange noise in every call

Hello everyone,

That is my first post here so please tell me if I don’t provide you with enough information on my asterisk setup.

I installed an Asterisk 1.4.24 with a DMlink DM1600a card for a call center in China.
Here is a link on the Chinese Ebay-like website to buy the card :

My card has 8FXS and 8FXO. It is connected to 8 service lines and 8 analog phones.

The system is working well, the dialplan works, queues work …

I had echo on the 8 analog phones, the agents heard their own voice very loud, but without delay, so I turned on echo canceling in zapata.conf.

The call center has been answering calls for 2 days now with this new pbx, and every agent reported noise on the beginning of many calls.

I turned on monitoring to get this noise recorded. It is very short, less than 1 second, and like 20 very quick taps. It happens twice very often in the beginning of some calls, and sometimes in the middle of calls.

You can download the sound here :

I first thought it was because of echo canceling. I turned it of and it didn’t get rid of the noise.

I checked the IRQ. My card was sharing one with an other piece of hardware. I changed in the bios.

lspci outputs this :

00:0b.0 Communication controller: Tiger Jet Network Inc. Tiger3XX Modem/ISDN interface Subsystem: Unknown device 8151:0002 Flags: bus master, medium devsel, latency 32, IRQ 18 I/O ports at e400 [size=256] Memory at ed104000 (32-bit, non-prefetchable) [size=4K] Capabilities: [40] Power Management version 2

So my card is using IRQ 18, and it is alone on this IRQ :

cat /proc/interrupts | grep 18 18: 57871772 IO-APIC-fasteoi dmtdm

I am out of ideas, I will also contact the card vendor, and try with an other card.

Do you have any idea ?

I found another clue about my problem.
I have been listening to calls in the call center with the Zapbarge application, and opened the asterisk console full of debug.
It seems that when a new call from outside begins to ring a new channel, some call center agents that were already using other channels hear the tapping noise. At the same time I get this on the console :

[Jun 4 09:23:06] NOTICE[452]: chan_dahdi.c:6836 ss_thread: Got event 18 (Ring Begin)... [Jun 4 09:23:06] NOTICE[450]: chan_dahdi.c:6836 ss_thread: Got event 2 (Ring/Answered)...

Anybody knows about a problem in dahdi that could create this noise ?

The DMLINK card vendor told me it might come from echo cancelers, but I don’t understand why echo cancelers would tap 5 minutes after the beginning of a call.

Excessive side tone suggests a hardware or cabling fault.

I suspect the strange noise is the result of a rather slow ring trip at either the central office, on the FXS ports, causing ringing currrent to be sent to the off hook phone. (Ring trip means the logic that turns off the ringing current when the phone goes off hook.)

Hearing this on another line suggests hardware problems.

Can you confirm that you are using twisted pair for all the cabling, either with separate cables, or with both wires from the same pair for each phone. However, simply mixing wires from different pairs shouldn’t cause excess side tone. I think you would need an indirect return path for a cabling fault to do that.

thank you for your reply david55,

It might be a cabling fault as you say. The 8 lines coming from outside are two RJ45 cables with 4 twisted pairs in each cable. Those 8 pairs are then plugged into the analog card.

From the analog card I have 8 other pairs (some twisted some parallels), gathering into two RJ45 cables going to the call center and splitting again to 8 twisted pairs plugged into the phones.

I don’t understand the " indirect return path ", could you explain how I might have done it ?

Is there a way to test the quality of the cabling ?

Before using asterisk, they were using a Panasonic Hybrid, with the same cables from outside and to the call center. They never heard of this tapping noise before, and they never had this much side tone.

This may help for the side tone:

By indirect path I meant that if one side of the pair doesn’t actually follow the other, but, for example, goes through the system ground. It’s probably unlikely.

The noise may be the result of the card (or dahdi?) not muting the connection until ringing is removed.

Would it be possibe to try a different card? Preferably one from Digium or Sangoma? If the same cables were used on a different system that leads me to a hardware issue in the server, could be the card or the server itself but most likely is the card.

Sounds like the caterpillar drive system in the red-october is running a little low on oil.

In all seriousness though what you’re hearing is ringing being switched onto the line, and then the ringing sound. I would troubleshoot this by, one at a time, disconnecting 1 of your FXO lines and then running the center. If the sound goes away, your culprit is that port on the card otherwise repeat until you have gone over all 8 ports.

This should find your offending line and/or port. If everyone hears this when a call comes in on the line, you probably have bleed-over in the card from cheap-craftsmenship and/or crappy isolation of the AC ringing.

Hope this helps


As Mark said this is ring tone you can hear.

It could be the cabling but more than likely just cheap hardware.

disconnect all the FXS lines except 1 and all the FXO except one.
configure it so that a call is sent to a unplugged fxs port and pickup a handset and break dialtone and see if you hear the noise , if you do its the card or possibly earthing, and I would try connecting/checking that the card/chassis is earthed.


Cheap hardware? in China? The world has gone mad!

Thank you for all your replies, it helped me much in my discussions with DMLINK.

After many tests and 3 weeks of intensive use of this analog card in the call center, I discovered many other problems.

1 - The tapping noise described before, seems to be a ring tone with an unusual period and amplitude.
2 - The second line on the same module can hear the first line ringing tones. This seems to be an electrical problem. DMLINK assures me that they don’t have this problem. We bought 3 cards, same problem on every module of each card.
3 - The busy detection or hangup detection is not correct, resulting in some trunk lines calling indefinitely and unusable by the call center.
4 - Sometimes, between 3 and 10 times a day, the call center agents pickup a call from the queue and in place of getting a customer they hear a tone, so loud and high-pitched that they have to take out their headsets and hangup.

I hope all the problems are linked.

We decided to buy a Digium card to compare. We bought the Wildcard TDM800P with 2 Quad Channel FXO. All the agents use Zoiper SIP now. It seems after one day in the call center that it got rid of most of the problems except for the worst one !

Now we get it more regularly, like 10 times a day, this very acute noise, so disturbing for agents I fear it might affect their work. Most of the time, they answer a call from the queue, they can not hear the caller, and after 5 seconds they hear that exploding acute noise, take off their headset and hangup. Sometimes they get this noise immediately after answering the call.

I am in contact with an engineer from DMLINK, and an engineer from, digium distribution partner.

DMLINK thinks my PSTN lines have unusual busy and ring tones, I recorded the tones and they measured it :

Cadence: 1000 ms on , 4000ms off
Cadence: 375 ms on , 327ms off[/ul]

Could it be the source of my problems ?
Should I change this in indications.conf ?

My company’s call center is located in Beijing. We have 2 other offices in Wuhan and in Guangzhou. The calls to the other offices are redirected to the beijing call center, so that local customers only pay the local price, and not the inter-province roaming price. The problem might also comes from the redirection provider. I am actually investigating on this.

My fear, is that I have to set up the exact same call center in Algeria next week, with the same analog cards, Digium or DMlink, and that none of them are working fine :cry: .

Are we talking about ringing or ringback. I would have thought that we were talking about ringing, which is always less than 68Hz and typically 20 to 25Hz.

Knowing the spectrum of the noise burst may give a clue to its origin. In particular, if it is a pure tone, in the range 2 to 3kHz, it may be a network signalling tone.

The spectral view of the sound :

could it be network coming into my line ?

All the calls with this problem are redirected from other provinces to our beijing call center’s lines. Could it be something different when a call is redirected before coming into your line ?

This one was coming from a mobile phone.

In my previous company we were running a call center on asterisk. First we were using Sip phone. Every day we got some hangups and some blank calls .i.e some times agents were able to listen callers voice but agent voice was not transmitted to caller. then we moved to IAX. We used IAX accounts for agents and used MozPhone as we had to open some url to agents on every received call. It does not completely solved the problem but reduced the hangups and blanks calls to some extent. May be you can dig it more and get your problem solved.

Thank you for your reply adnanraza.

I had the same problem before with IAX, and phones plugged to FXS.
This problem is coming only from one type of calls, redirected from other cities to the capital.