GXW 4104 Incoming Call Billing


Im using a GXW 4104 as an FXO Gateway for my PSTN Line. Incoming and Outgoing calls have no problem. However, It seems that GS GXW 4104 cannot detect whether an outgoing call is answered or not. This means that an outgoing call is being billed while the initiated call is still ringing on the other end. Is there a setting I am missing here?


I am guessing the issue is that you are using a PBX and the PBX has a billing feature based upon time. In the case of outgoing calls, you are probably correct. The GXW is a gateway and executes the function requested by the PBX. The PBX sends a message that tells the gateway to seize a POTS line and once done to dial the number. This is an INVITE and the GXW will respond back with messaging to indicate the progress which will finally result in a 200OK (signifying a connection to the analog line, not to the other end) once the number has been successfully dialed. When the 200OK is received, it means that the analog line is now in use and open for the PBX to use or listen. Think of it as a telephone, you pick up the receiver, dial a number and wait for someone to answer. The telephone has no control over ringing or answer or other. It merely relays the signal and it is the provider who might bill. This is the case with the GXW. It merely has connected the line and is relating the audio and has no control. The call billing started the instant that the PBX sees the 200OK and it has nothing to do with a busy tone, ringing tone, congestion tone, etc. The GXW is only relaying what is on the analog line.

There is no way around it that I am aware.


Hi lpneblett,

Thanks for your reply, I got our explanation regarding this issue, I have a asterisk machine as my PBX system, However, along with my GXW, Im also using a GSM Gateway for cellular calls and I have no problem with the billing when these GSM Gateways are used for outbound calls. I read a thread here in the forums somebody suggested to enable Polarity Reversal on the GXW. I did that and got a 0 sec on the billsec but the problem is I can only ringing on the local phone that initiated the call. Is this maybe a Service Provider issue?


Polarity reversal work for provider not you, it OSTN option.
As i understand you collect CDR on asterisk, right ?

GXW have set
FXO Lines ⇒ Dialing =>Stage Method as 1 ?


It is a function of analog signaling and not polarity reversal. Polarity reversal is a disconnect signal and not associated with billing directly and not used by all providers. GSM is a digital function and not the same as analog. When the GXW connects to the analog line and there is dial tone, this is all the GXW knows and then dials. It did what the PBX told it to do and what the results are with regard to ringing and answering are solely up to what is physically heard on the line.

The only way that I am aware is if the provider supports metering pulses and you have the equipment to take the pulse, which the GXW does not. The pulse is sent by the provider and the device receives the pulse and represents a pulse as a billing increment.

If you must have more accurate billing then your only option is SIP or have the caller maintain a manual log. I do not know what your billing increments are, but others have simply taken the call time and subtracted one minute using the minute as a grace for the ringing period. I know of other that bill for the total time of the call using the rationale that they are still on the phone devoting their time to the client while the phone is ringing. This is not really different than being put on-hold by the client and having to wait in the meantime. You are still expending time on the client.

Other than metering pulses, I do not see away for you to use analog lines and know that the billing period should start only when the other side answers.


Hi Marcin,

Yes I am collecting data on asterisk CDR and Also yes I’m using 1 Stage Dialing method on GXW. Any suggestions?



I was also thinking of subtracting a certain amount of time (60 secs) to compensate for the time the phone rings on the other end. The issue I see with that option is what if the person on the other line immediately answered the phone. Lets say ring time 15 secs and talk time of lets say 10 secs for a total of 25 secs ring time and talk time combined. If I subtract that from 60 secs for the compensation if would appear that the call will not be billed at all.


4104 establish connection ?
AFAIR it should wait for PSTN ON just do it.

Maybe PSTN is pickup in auto even if call is not established, in that case it is Provider fault.


Marcin -

I think the Invite from the PBX is getting a 200OK from the GXW upon setting up the SIP leg to make the call and with the 200OK, the PBX considers it an answer and starts the billing. In the meantime while the connection to the far end has not been completed, the analog audio channel is open and delivering back to the GXW and the PBX, so the caller is hearing the ring, busy, error, etc., but as far as the PBX is concerned, the call is connected. It does wait for dial tone before dialing and I assume STage 1 given no mention of a second dial tone or dialing.

Aysman - as stated, you have choices. You can make it 60 seconds, 30 or whatever you want or even include the ring time or consider SIP.


@lpneblett maybe, i really do not remember but i think call is not established by 200 OK untill is answered, but maybe it is at first as you write. Only packet or syslog can show this.
For sure stage dial 2 make connection.


How would the GXW know if answered? The audio channel is already open delivering ring. The only CPC would be the disconnect signal. What signaling is conveyed in the audio path that the GXW will understand to be an answer signal?

I tend to think the same thing for stage 2 dialing. At that point the control has been given to the provider’s dial tone for DTMF pass thru entry from the phone/pbx. I think the GXW is completely out of the way at this point other than listening for a BYE or other SIP message and/or a disconnect signal from the other end.

I suppose it may understand certain tones or lack thereof such as dialing an unknown number that will result in an error message and then perhaps that would confirm my thought if it shows talking at the PBX log. Perhaps even dialing a number that you know to be busy would be interesting.

I may test it if I can dig a GXW up.


You are not charged in PSTN network if call is not established.
Dial and ringing tone is specific. Well but really depend :slight_smile:
It can be as you say that GXW establish call just after PSTN line start.


That is true in a PSTN network when the billing is done by the carrier…not by a PBX. As the provider switch senses the off-hook condition, it knows answer and starts the meter.

I did do the test using a UCM and a GXW4108:

I am showing the UCM to GXW leg. I connected the GXW to a line simulator. The CDR reflects:

. The 10 seconds of ringing is inclusive of the leg to/from the UCM and GXP2200.

I waited some 30 seconds before answering the ringing analog phone.

The results for stage 2 dialing are even quicker as it starts with the 2nd dial tone.


Wireshark cut to short, no length of call. But still it start after 6 seconds.
There is no way to determine real time table of this call without carrier data.


There was no need to keep the call running in wireshark as the effort was to find out when the system implemented the answer part and thus the billing. Without a doubt, it starts on the 200OK which is when the gateway does the connection to the analog line.

Out of curiosity I looked around quite a bit and found a number of older documents (40’s thru mid 70’s) talking about telephone metering pulses for billing, but nothing as of late.