[cisco-voip] no Ringback on 7940-SCCP when calls are going over SIP

Wed Sep 17 13:17:52 EDT 2014

That’s one nice detailed write-up. While on this topic I thought why not add a bit more information future reference on this thread. Allowing bi-directional media setup upon receipt of the 180/183 with SDP in a delayed offer call-flow requires that:

1.       Disable Early Media on 180 is unchecked (default is unchecked)

2.       PRACK enabled at the SIP Rel1XX Options dropdown

Doing this provides CUCM with a method for responding to the SDP offer (inside the received 1XX) with an SDP answer (carried by its PRACK). Without PRACK, and assuming Early Offer is disabled, CUCM has no SIP method for carrying the SDP answer in response to the 180/183 offer. The only SIP method CUCM can respond back with an SDP would be an ACK, but this would be in response to the 200 OK – too late for an Early Media call.

The getPort capability Anthony described is spot on and I also highly doubt the 7940/60s support this method. A scenario where the getPort capability would be seen:

1.       SIP trunk is configured for Early Offer (insert MTP if req)

2.       Phone running SCCP v20 or up

3.       Outbound call is attempted

In this scenario, CUCM will attempt to send an INVITE with an SDP offer containing media information of the phone. The getPort capability allows CUCM to request for this media information from the phone BEFORE media turn-up and will insert the information gathered from the phone into its INVITE SDP offer. For Early Media to be successful, the remote SIP UA will then need to send a 180 or 183 w/ an SDP answer to complete the offer/answer exchange. If an IP phone doesn’t support getPort (PortReq & PortRes events), then Early Offer will force CUCM to allocate an available MTP resource for gathering media information to insert into the INVITE which should also use PorReq. But I would be surprised if local ring back wasn’t provided even if the phone doesn’t support PortReq.

A quick test shows that in a delayed offer call with a received 183 w/SDP and no PRACK, a StationD AlertingTone event is sent to a 7960 phone for local ring back. In this case the “Disable Early Media on 180” option is at its default (unchecked). In an Early Offer [attempted]call, the 7960 doesn’t support getPort, an MTP resource allocation is attempted but fails, and the call proceeds as a normal Delayed Offer using AlertingTone to play local ring back to the phone. With PRACK enabled, standard media turn-up occurs and PRACK contains the SDP answer. If the recommendation below doesn’t resolve the issue, I’m really curious to see what detailed CCM SDL traces would show.

Hope this helps!

Dan

From: cisco-voip [mailto:cisco-voip-bounces at puck.nether.net] On Behalf Of Anthony Holloway
Sent: Wednesday, September 17, 2014 10:43 AM
To: Nathan Reeves
Cc: cisco-voip at puck.nether.net
Subject: Re: [cisco-voip] no Ringback on 7940-SCCP when calls are going over SIP

There have been some good answers so far. I'd just like to offer the same possible explanation, just worded slightly different.

SCCP phones which are SCCP version 19 and lower, do not support the getPort message, and therefore cannot offer up a UDP port number to CUCM early enough to allow CUCM to send Early Offer over a SIP trunk without the need for an MTP.

http://www.cisco.com/c/en/us/td/docs/voice_ip_comm/cucm/rel_notes/8_5_1/delta/delta.html#52746

Which phones and phone firmware combinations get you SCCP v20 support is a bit unclear as seen below.

https://supportforums.cisco.com/discussion/11543476/sccp-phones-sccp-v20-support

Ringback tone is not generated locally, or cut through from the remote side, until a progress message is receive by CUCM from the SIP trunk.  This is typically a 180 or 183 response code.

There's really only two ways to go about ringback in this situation.

1.) If CUCM did in fact invoke an MTP to be able to send Early Offer, and the SIP trunk responded with their Answer via a 1XX response code, and CUCM sent a PRovisionalACKnowledgement which got ACKnowledge, and the far end device was playing ringback (or some kind of audio), then the SCCP phone should have heard ringback (or some kind of audio) from the far end device.

That's a big if, with lots of conditions.

2.) The other way, is to disable early media support on the SIP trunk all together, so CUCM just instructs the phone to play ringback locally.  You can do this as a test:  On your SIP Trunk's SIP Profile (make sure it's not being shared with other devices as to not affect them), check the checkbox for Disable Early Media on 180, save then reset your trunk.  Call will drop.  I've seen CUCM gracefully tear down calls by sending out BYE's to all active dialogs, and then drop the trunk.

Help on that setting says it all:

By default, Cisco Unified Communications Manager signals the calling phone to play local ringback if SDP is not received in the 180 or 183 response. If SDP is included in the 180 or 183 response, instead of playing ringback locally, Cisco Unified Communications Manager connects media, and the calling phone plays whatever the called device is sending (such as ringback or busy signal). If you do not receive ringback, the device to which you are connecting may be including SDP in the 180 response, but it is not sending any media before the 200OK response. In this case, check this check box to play local ringback on the calling phone and connect the media upon receipt of the 200OK response

Note     Even though the phone that is receiving ringback is the calling phone, you need the configuration on the called device profile because it determines the behavior.

If that does end up fixing your issue, you just have to decide if early media on that trunk is a requirement or not.  If so, either replace your phones with newer phones (SCCP v20+) or as the far end if you can create a separate trunk to them, specifically for this work around, and then via CSS/PT you steer these legacy phones over the work around trunk, while everyone else uses the existing trunk.

As for why it only start when you moved to CUCM 10x....I'm not certain.  During an upgrade, settings usually just carry over.  And Cisco is not in the habit of turning on new features or nerd knobs which would contradict early default behavior (I.e., If early offer was off by default in 8.5, it would not be on by default in 10.5).

On Wednesday, September 17, 2014, Nathan Reeves <nathan.a.reeves at gmail.com<mailto:nathan.a.reeves at gmail.com>> wrote:
Anyone come across anything like 7940's not playing Ringback to the user when an external call is made out externally over a SIP trunk.

CUCM is V10.  Firmware on the 7940's is as up to date as possible.  Calls proceeded normally on CUCM 8.5 normally.  Only noticed this on the move to V10.

Thanks

Nathan
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