DTMF Keypad Capture
DTMF keypad capture securely suppresses telephone keypress tones before they reach agents or call recordings.
How It Works
When a call enters secure capture mode, Subphonic's telephony proxy sits inline with the call media path. As the customer enters their card details via keypad:
- The DTMF tones are detected in the RTP media stream before reaching the agent.
- The tones are suppressed — replaced with a neutral signal — so neither the agent, the call recorder, nor any downstream system receives the raw digits.
- The digit values are routed securely to the Transact capture session only.
- The agent's screen shows real-time capture progress (asterisks per digit entered) without ever exposing the actual card number.
DTMF Signalling Modes
Modern telephony systems carry DTMF in more than one way. Subphonic Transact handles all standard modes:
| Mode | Description |
|---|---|
| RFC 2833 / RFC 4733 | DTMF transmitted as named telephone events in the RTP packet stream, separate from the audio payload. The most common mode in VoIP environments. |
| SIP INFO | DTMF digits signalled as SIP messages in the signalling channel, entirely outside the media path. |
| Inband (Audio) | DTMF encoded as audible tones directly within the RTP audio stream. Less common in modern VoIP but still present in legacy and PSTN-connected calls. |
Inband DTMF is a special case. Because the digit tones are embedded in the audio rather than as a separate signal, standard suppression alone is not sufficient. Subphonic's Inband Media Detection capability monitors the call audio in real-time and alerts the agent if inband DTMF is detected during a secure capture session.
PCI DSS Scope Impact
When DTMF suppression is correctly deployed:
- Agent environment — Agents never receive keypad digit values via their audio feed. Their device, network, and workspace are taken out of PCI scope.
- Call recordings — Recordings contain only neutral audio during the capture window; no recoverable card data is present.
- CRM and CCaaS systems — Systems that receive the call audio stream receive suppressed media, keeping them out of scope.
- Telephony infrastructure — Only the Subphonic Transact components that handle the secure digit capture are considered in-scope.
RFC 2833 / SIP INFO suppression is handled automatically at the signalling layer. Inband DTMF requires media monitoring — ensure your deployment has Inband Media Detection enabled if your telephony environment may produce inband DTMF.
Correlation
Before capture can begin, Transact must link the agent's browser session to the correct telephone call. This is called call correlation and is a prerequisite for DTMF secure mode activation.
- Automatic — The parent application (e.g. Teams, CCaaS desktop) passes call metadata to Transact on launch.
- Manual (DTMF) — A short generated code (e.g.
#974##) is displayed to the agent and typed into the handset to identify the call. - Manual (Search) — The agent searches active calls and selects the correct one.
Once correlated, Transact instructs the telephony proxy to enter secure mode for that specific call.