[VoiceOps] SIP return message on misdials - 487 or 500/503 or ?

Ivan Kovacevic ivan.kovacevic at startelecom.ca
Tue Oct 13 00:43:41 EDT 2015

Except… that in case of call centre traffic, this leads to repeated
dialling of invalid numbers, leading to a ridiculous number of unnecessary

It is a pet peeve of mine and I wish there was a way to fix it. So that
invalid numbers can get a permanent failure result and stop clogging up our
network and ruining the connect rate.

To illustrate the issue… if a call centre starts calling a monthly list
which has 10-15% invalid numbers (typical). By the end of the month, they
will be calling those numbers daily (or several times per day, since 503 is
an error, not a valid attempt)… and as they deplete the list, those invalid
numbers will be dialed more and more frequently. If they don’t have a
maximum attempt limit, you are really screwed.

Even if they do, and is set to 10 or 15, these will be dialed the full
number of attempts... If all other records complete in 3-4  attempts on
average, these will go the full 15 (because of 503 or other temporary
failure response)… so in the end… those 10% of invalid numbers will result
in a much larger % of (unnecessary) dials.

I think this is one of those things that’s waiting for a standard or a
smart provider to come along and fix it. I’ve toyed with the idea of
letting our CPA resources run against all 503s to see if we can sort out
some of these based on early media/tri-tone/voice reject. Just haven’t had
time to do it… and we do what everyone else does… just let the clients dial
these and pass the crap to our downstream providers…

In the meantime, we tell our call centre clients to go through their lists
in power/preview mode towards the end… so they could scrub these NIS
records. This is a half-ass solution in my opinion, but without a better
one… it’s all they can do.

I’ve tried raising the issue with our providers as well, to try to get
better (more accurate) SIP responses… but here isn’t much they can do
either… so… here we are.

Best Regards,

Ivan Kovacevic

Vice President, Client Services

Star Telecom | www.startelecom.ca | SIP Based Services for Contact Centers
| LinkedIn

*From:* VoiceOps [mailto:voiceops-bounces at voiceops.org] *On Behalf Of *Calvin
*Sent:* Monday, October 12, 2015 3:46 PM
*To:* Voiceops.org <voiceops at voiceops.org>
*Subject:* Re: [VoiceOps] SIP return message on misdials - 487 or 500/503
or ?

+1 on Glen's answer.

Intermediate carriers deal with enough FAS and other garbage that they
cannot trust every 404 or 604. We've determined that it's better to 503 a
possibly invalid number for the customer to later retry, than falsely 404 a
valid number and get complaints about how we trashed their expensive number
list. So long as there is no pre-session media before the 404/604 (i.e.
180/183 tri-tone, voice reject, busy, etc.), we and/or the customer can
still route advance and potentially connect that call.


*Calvin Ellison*
Voice Services Engineer
calvin.ellison at voxox.com
+1 (213) 285-0555

*voxox.com <http://www.voxox.com/> *
9276 Scranton Rd, Suite 200
San Diego, CA 92121

[image: Voxox]

On Mon, Oct 12, 2015 at 12:11 PM, Glen Gerhard <ggerhard at sansay.com> wrote:

Hi Erick,

the only carrier who knows the number is invalid is the LEC/CLEC that owns
the block.  They probably will send back a 404 Not Found or 604 in most
cases.  However unless you are directly connected to them you may not get
that response returned to you.  The intermediate carriers will generally be
configured to return a 503 in case of any failure.

Also the Cancels you see may be due to the termination carrier playing an
inband ring media that is a tri-tone "your call cannot be completed as
dialed" message.  Since it is during the ring phase the caller (or dialer
app) will Cancel the call resulting in a 487 cause code.  For dialer apps
this is a benefit since the good ones will recognize the tri-tone and
remove the number from their calling list.


On 10/5/2015 9:19 AM, Erick Bergquist wrote:

Thanks for the replies.

On the cause 41 I get a 503 which matches up.

Getting cause 63 back for the ones where I see SIP 500 Internal Server

Failure which is default behavior.

 I guess I'm good then, just wish I wouldn't see 5xx level for

mis-dials. Yes, the numbers are valid E164 formatted numbers for US

which the provider wants to see in that format.   I can't call these

numbers from other sytems either so they are indeed bad, un-allocated


Thanks, Erick

On Mon, Oct 5, 2015 at 10:51 AM, Jay Hennigan <jay at west.net>
<jay at west.net> wrote:

On 10/5/15 8:16 AM, Erick Bergquist wrote:


Just trying to get idea of what is normal on what providers should

return for a misdial, bad unknown number, etc.

On one provider, I get a CANCEL and a 487 Request Terminated on mis-dials.

On another provider, I get a 503 Service Unavailable and a 500

Internal Service Error back.

It depends. CANCEL and 487 Request Terminated typically comes from the

originator and means that you hung up before the call completed. Carrier was

in post-dial delay and hadn't returned anything (yet).

503 means that the carrier can't or won't process the call. Could be a

misdial where the number can't be parsed as in not enough or too many

digits, prefix starts with 1 or 0, etc. Could also be that you tried to dial

a valid number that the carrier doesn't handle, such as international,

900/976, etc. Could also mean that you didn't pay your bill or are coming

from an IP address that isn't a customer of that carrier.

For a number that is in the correct format but isn't in service, you might

see 503 or also 404 or 604.

500 internal service error is usually a technical problem with the carrier

and not a misdial.

Different carriers map ISDN/SS7 cause codes to SIP differently. See:



Jay Hennigan - CCIE #7880 - Network Engineering - jay at impulse.net

Impulse Internet Service  -  http://www.impulse.net/

Your local telephone and internet company - 805 884-6323 - WB6RDV


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