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Crosstalk - VoIP

Level 1
Level 1

Okay hear me out.


I'm wondering if the collective hive mind here could help me out explaining this in a manor that anyone could understand . Customer has an issue that the far end (calling party) can hear MoH when they are put through to an agent. Not that they hear just MoH, they can also hear the agent talking so MoH and agent audio at the same time.
Now this is a fairly large customer and for this sector they have no control how calls come in. It's centrally managed and there are IVR, prompts, etc before it reaches the part I manage. The important part is, when it reaches and exits the bit under my control it is SIP end to end. Even on the gateway (CUBE). I've explained that it's pretty much impossible for crosstalk of hearing another audio source at the same time is impossible unless there is a problem with DSP which aren't being invoked. I could understand if you heard music on hold and then the agent or visa versa but never both at the same time in IP. I understand hissing, crackling, etc could occur if the cables were faulty but never clear MoH and agent at the same time.
Now I understand that this sort of cross talk can occur on electrical circuits but not digital. The users I am working with cannot confirm if at some point further in the call it breaks in or out on POTS. Now they won't go to the team that manages anything passed what we manage until we can provide it's definetly not our equipment. I don't know how I can explain myself better, they are requesting somewhere in a SIP RFC why this cannot occur but it's more of why it occurs in electrical and not digital. Could someone maybe link me and explanation to why this kind of crosstalk doesn't occur in digital. All I can think of this point is pumping all calls to a recorder from the CUBE but theyll likely not pay for that and thus I'll be going around in circles and I'm clearly not getting my point across properly to the customer. Sorry if this seems like a silly question and thank you.

3 Replies 3

Adam Pawlowski
VIP Alumni
VIP Alumni
I have absolutely no idea how you could pull that off without calls being bridged into conference resources. The agent could hear this on a Cisco IP Phone if the phone were told to listen to media and it were being streamed in - it could be mixed but that’s very unlikely. I suppose if someone implemented multicast MoH very wrong you could get something bizarre happening as well.

I’d bet on there being an analog component somewhere. A tapped analog recorder with wires on top of unshielded 70V PA BGM, the agent playing music on the computer hooked up incorrectly somehow, etc.

I don’t think it’s strictly impossible to have media mix in even with SIP, just very unlikely.

I have run into a similar symptom once before for an all-SIP / IP call. In that instance it was an early offer call and a routing issue was preventing the return traffic (eg 100, 183, 200 OK) from one of the CVP media servers making it back to CUBE... so CUBE sent the INVITE to the next dial peer with the same SDP, including the same UDP receive port for RTP. Since CVP got both INVITEs it dutifully began transmitting media for what it perceived as two calls. CUBE forwarded both sets of RTP toward the provider and - viola - two audio streams were heard be the calling party.


your thoughts please. we are experiencing "xtalk" on our 11.6 cucm using Verizon SIP. the strange thing our internal users are the only one to hear it and it is always the UNITY greeting being played??  very strange and extremely hard to isolate. but if you had any thoughts I would appreciate hearing them.


Thank you