Digit Signaling In Cisco IP Telephony

Digit signaling is the way digits are sent from an IP phone to CUCM. We will explore both an SCCP and a SIP phone.

Digit signaling in a Cisco SCCP phone

We will expand on the concepts of digit-by-digit signaling in the SCCP Digit Collection paragraph of the CVOICE Foundation Learning Guide, 2nd Edition.

First, we start with a Cisco 7911 SCCP phone.

We will use Wireshark to see which packets are exchanged. Here is the network setting:

digit-signaling-cisco-telephony----2016-03-26 10_40_54

We run Wireshark. Initially the phone only sends keepalive messages to CUCM, at regular intervals. CUCM replies with KeepaliveAckmessages.

digit-signaling-cisco-telephony----2016-03-26 10_28_47

Each action is translated as an instruction to CUCM. When we press on the New Call soft key, the phone sends the SoftKeyEventMessage. The CUCM replies with:

  • SetRingerMessage which instructs the phone to play the ringer file stored on the phone.
  • SetSpeakerModeMessage
  • SetLampMessage, which lights up the button next to the current line
  • CallStateMessage

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I press on one button. The phone sends a KeypadButtonMessage. CUCM replies with StopToneMessage and SelectSoftKeyMessage

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The same messages are exchanged when I dial the second digit

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So we see how Cisco 7911 phones use digit-by-digit signaling.

Digit addressing in a third-party SIP phone

With a third-party SIP softphone such as Express Talk, the digits are sent en bloc. As demonstrated by the screenshots below, no digits are sent unless I press the dial softkey.

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The digits are sent en bloc in a SIP INVITE message.

digit-signaling-cisco-telephony----2016-03-27 15_11_47

You will find more articles about the CCNP Collaboration track here

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