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:

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

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

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

The same messages are exchanged when I dial the second digit

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.

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

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