Ring Back while 183 Session Progress

[JustForVoice_2]

Hello,

I have an issue when I try to call from IP Phone to Telco SIP provider

IP Phone >> SIP Trunk>> CUBE>> SIP Provider.

The issue is I receive a message from Telco 183 Session Progress. But the IP phone rings. It rings even if I call to my cell phone and it’s off.

I checked the incoming and outgoing dial-peer and I added this command:

progress_ind alert strip

disable-early-media 180

But the same issue.

[Mohammed al Baqari]

Hi,

Couple of points here

* The command 'progress_ind alert strip' has no impact. This is use in H323 and ISDN

* 183 Session progress will always trigger early-media cut-through as it comes with SDP content. The command 'disable-early-media 180' has no effect in this case as this will impact 180 ringing message

* Once early-media takes place, ringback will be provided to the phone from ITSP

* This is no feature to disable early-media cut-through for 183 Session Progress

You need to speak with your ITSP to provide the right announcement once early-media is established. They shouldn't play ringback regardless of the called party status

[JustForVoice_2]

Thank you Mohammed (+5)

I attached an image for the call flow, to make sure that I describe the problem well.

[Vivek Batra]

Hi,

Yes, you described it well.

If you have shared complete call flow, I doubt media (RBT) is sourced from service provider because SDP negotiation is not completed between CUCM and CUBE, there is only 183 with SDP offer is sent by CUBE to CUCM and there is no further SDP answer and hence media with ITSP can not be connected.

I suggest to do either of the following step and then share the result;

1. Enable SIP early offer in CUCM, or

2. Enable PRACK in CUCM.

- Vivek

[JustForVoice_2]

Thank you for your help (+5) I tried to enable SIP early offer in CUCM but I found multiple options under the sip profile, could you please tell me how to do this? For PRACK I did not find any parameter :)

[Vivek Batra · Accepted solution]

Hi,

1. Early offer support for voice and video calls under SIP profile.

2. For PRACK, SIP Rel1XX Options under SIP profile.

- Vivek

[Mohammed al Baqari]

Hi Vivek,

Correct me if I am wrong:

- CUCM won't send PRACK unless it receives 100Rel Required or Supported in INVITE. Therefore, turning on this option won't guarantee sending PRACK (we need to see if INVITE message contains 100Rel)

- For early-media cut-through you don't need to see return message from CUCM for 183 SDP. Once CUCM receives 183 SDP, it will cut-through the phone.

Therefore, I still believe that the ringback is coming from ITSP   not CUCM. However, the testing will confirm.

[JustForVoice_2]

Thank you Vivek and Mohammed for your support

[JustForVoice_2]

Hello,

I got a complain from my customer that if they call mobile phone and the called party did not answer the call, the system will redial and make the call again.

Is there any relation between this issue and the changes that I did:

changing Rel1XX to send PRAK if 1xx contains SDP

Regards,

[Vivek Batra]

Hi Mohammed,

It is outgoing call hence CUCM is sending INVITE, not receiving. By default, PRACK is disabled under SIP profile hence we can safely assume that 100rel would not have included in INVITE.

Now CUBE is sending 183 session progress with SDP. Since PRACK can't be used for this call and SDP answer can not be send by CUCM in PRACK, I suspected that CUCM can interpret it as 180 Ringing and instruct phone to supply local ring back.

One thing I thought to ask before whether 183 Session Progress message had included 100rel support or not, but didn't ask considering the fact that SDP offer can not be included in unreliable provisional response and if it is included in 183 Session Progress, it would have transmitted reliably only and hence with 100rel. 

Most important, since there was no SDP answer from CUCM to CUBE, this means that CUBE doesn't know where to send RTP packets (if coming from ITSP) hence for me, it was not possible in this case for ITSP to source RBT to phone.

- Vivek

[Mohammed al Baqari]

Thx Vivek for the clarification

[Vivek Batra]

My pleasure Mohammed.

- Vivek

[JustForVoice_2]

I fixed the issue after changing Rel1XX to send PRAK if 1xx contains SDP

[Vivek Batra]

Thanks for updating the result.

- Vivek