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Early Media, aka in-band information, is sent from the network to the calling phone in some circumstances, to provide alternative tones or extra information about the status of the call. In GSM this is available because of "Early Assignment" of the TCH. This early assignment is also possible in the called phone, but I have never heard/read of a use for it, besides the initial purpose of avoiding delays.

Now, question is, does assignment of the TCH implies the capability of receiving early media FROM the mic of the phone, before the actual start of the conversation? And if the capability exists, has it been used? Perhaps in outgoing calls there were plans to offer *44, voice activated dialing, services and then it is a sort of legacy.

Of course the worst possible case is the inbound call: TCH and microphone open in the called phone without actual ringing of the signal (call, abort with silent notification and leave channel open until the timer expires, then call again).

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    From my understanding there are some reports that the mic recording is sent before the phone accepts the call. But only in the following models: Nokia X2-01 (05) and C3 - this is based on information that I've found I haven't made any tests yet. Not sure if the Network Operators have this feature enabled, or this can be changed at MS level (different manufacture). Have you made any research on this topic?
    – Etraud
    Commented Jan 15, 2017 at 18:48

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Yes, it is possible for some combinations of operators and some mobile phones.

Here is a demonstration, including a video and detailed instruction how to reproduce (in Czech language with Czech SIM):

Problem demonstration

Czech telecommunication office reply

Short translation: Nearly 100% reproducible when calling from a Czech leased line to an old Nokia with Vodafone CZ SIM card as a receiver. Also calling from UPC leased line works. Also calling from some VoIP phones work. In one case, it was reproduced when calling from a mobile phone with T-Mobile SIM. Using VoLTE disables this feature.

Czech telecommunication office says it is a correct behavior of the network and it will not be fixed.

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  • Nice! I wonder is it is really one-way only or if the return way fails to return to the caller but still arrives to the central (where the old switchboard operator could be hearing).
    – arivero
    Commented Jan 3, 2018 at 15:11
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    Its purpose is probably different. See RFC3960. It makes possible to implement tone dial.
    – Stanislav Brabec
    Commented Jan 3, 2018 at 22:03
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    And yes, it seems to be one-way. The recipient device should not initiate the media channel before the call is accepted.
    – Stanislav Brabec
    Commented Jan 3, 2018 at 22:05
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+50

I don't believe that this is possible, as a standard/recommendation, due to the separation of signaling from media circuits. The process of answering a call is separated from the interconnection of the circuits, and the latter is only possible/feasible after the call-status of acknowledgement of an answered call is returned.

The ringing heard on the caller side isn't necessarily indicative of the call-progress as it can and is often first provided by the originating network (you've heard double ringing before?), and the custom ringtones/messages are from a in-network peripheral that the called device is unattached from other than by account configuration stored separately from the physical device.

However, as a customization, this is not really a challenge for a non-standard device to capture the audio from the microphone of the pre-ringing device and send it back through the network as MMS (SMS-core extension) content.

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  • I think that the question really goes, "is early assignment still a thing in the current deployments?", and "what does it imply?". You seem to tell that EA does not connect the mic/media to the call circuits. Not so sure when you see that cheap phones put the mic line straight into the codec or modem chips.
    – arivero
    Commented Jan 20, 2017 at 20:08
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    In GSM very early assignment is actually possible, and you are correct that if a poor/malicious modem was installed, media could be delivered between the call being accepted and the callee alerting. It is also possible in IMS; I believe that since in the IMS call setup that the user plane resources are actually implemented just before the alerting to the called device, if there was a way to delay/cancel the alerting one could expect a supporting device to allow mic audio to pass through to the originating device. Very interesting. The only legitimate use is spying, I think.
    – Lance
    Commented Jan 21, 2017 at 18:25
  • Another use that I thought of is allowing for short media based responses to incoming calls without answering them. The called device could allow the called party to record a quick audio or video response to the incoming call, but without actually answering the call. That message could be delivered to the calling party over those user plane resources.
    – Lance
    Commented Jan 23, 2017 at 16:35
  • yea, that usage is near of the "legacy" that I think motivated the ambiguity of the standard: have an switchboard operator to manage the call and recognize "voice orders" in both extremes before the actual start of the call.
    – arivero
    Commented Jan 24, 2017 at 13:13

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