Problem with IVR and "ACK is not received from sip"

[b.ebner]

Hello there,

I recently noticed some problems with the IVR and Incoming calls from a mobile phone.
The Problem is the following:

We do have an external Number that ends with "00".
There is an incoming rule that directs this Number to an IVR on internal extension 801.

Our Provider is Telekom.
The provider of the mobile phone is also Telekom.

When I call this number from my landline phone everything seems fine.
When I call from the mobile phone it sometimes happens, that the call ends immediatelly without the IVR responding.

In the logs I can see the message "ACK is not received from sip"
In the call list I can see that the mobile phone was connected 32 seconds with the IVR.

After 32 seconds a call is ended if no ACK is received.

Strange is that the phone wasn't even connected with the IVR but the 3CX phone system thinks that it was for 32 seconds.
That leads to a disconnected call due to no ACK is received.

But there can't be an ack received when the mobile isn't even connected.

Any ideas?

Bjoern

 

[leejor]

The 32 second timeout is typical if there is no ACK received. The question is, why is it not being received for one (type) originating call. Have you determined if it fails on calls from one mobile provider, or all? If you can determine a pattern, (calls fail from only originations from one mobile company), then it's time to involve your VoIP provider, and present your findings. Let them determine why the call are failing. They may have an issue on trunks from a certain origination.

 

[b.ebner]

I know that a timeout after 32 seconds is normal if no ACK is received.

The problem is that ends the call withon about 2 seconds after calling without answering at all.

So the mobile phone wasn't connected for 32 seconds at all but it shows up in the call list like that.

If I call a normal extension from the same phone everything seems fine.

I think the problem is that the 3CX thinks that the call is already connected for 32 seconds but it didn't connect at all.

So there was no chance to send an ACK.

 

[leejor]

Ok, if a call from the mobile works, to another extension, but not , to the IVR, then compare the 3CX Activity logs of both calls. Also , compare a call from a landline (that works), to the IVR, with a call, from the mobile, that fails.

 

[b.ebner]

Strange is that it sometimes works from this mobile and sometimes it doesn't.

It's a fifty fifty chance.

Only difference for a working and not working call is, that ack is not received and it doesn't show the right connection time when it doesn't work.

 

[YiannisH_3CX]

Hi @b.ebner

Run a wireshark capture to see what is happening. You should see the call hitting the PBX and the duration of the call before it drops as well as any messages sent in between that could cause the issue.

 

[b.ebner]

I did a capture of a problematic call:

"Startzeitpunkt","Endzeitpunkt","Initiierender Sprecher","Von","Nach","Protokoll","Dauer","Pakete","Zustand","Kommentare"
"2020-04-13 10:25:17","2020-04-13 10:25:25","x.x.26.229","<sip:[email protected];user=phone>","<sip:[email protected];user=phone>","SIP","00:00:07","40","CALL SETUP","INVITE 200 200 200 200 200 200 200 200 200 200 200 200 200 200 200 200 200 200 200 200 200 200 200 200 200"

As you can see the call didn't last for 32 seconds but call list does show that it lasted for that time.
There seems to be even audio (the IVR) in the capture, but the mobile phone can't even establish a proper connection.

I attached the call flow.

Any idea what could cause this?

 

[YiannisH_3CX]

I can see the system sending 200 OK messages without reply but your capture looks incomplete as there is no indication as to who is terminating the call. There should be a BYE message in the capture that will tell us who is terminating the call.

 

[b.ebner]

You are right.
I didn't wait for 32 seconds with the capture.

My mobile phone immediately disconnects but the call itself seems to go on for 32 seconds.
As my mobile phone is not connected at all there seems to be no ACK.

I attached the complete call flow of a new capture.

 

[YiannisH_3CX]

Now it looks better. From what is see is that the call hits the system and it is answered by the PBX. As you can see the PBX keeps sending 200 OK messages to the provider and is not dropping the call for 32 seconds. Once the 32 seconds elapse the PBX sends a BYE message to terminate the call.
As you can see the provider is replying to the BYE message but not the 200 OK messages.
I cannot see what is included in the messages but is suspect the either the provider does not receive the 200 OK messages so there is no reply or the provider does not like the 200 OK messages which results in them hanging up the call with you but not replying to the system.
I believe you need to speak to your provider regarding this but i can take a look at the actual capture if you'd like. Just send me a p.m. with a link so i can download it and have a look.