Each key on a phone keypad sends a DTMF tone: two frequencies played together, one for the key's row and one for its column. Phone systems decode these tones to know which key was pressed. That is how callers move through an IVR menu ("press 1 for sales"), enter an account number or a PIN, or reach an extension.
On VoIP calls, DTMF can travel in three ways:
- In-band: the tones stay in the audio. Simple, but compressed codecs such as G.729 can distort them until they are no longer recognized.
- RFC 4733 (formerly RFC 2833): key presses are sent as separate events next to the voice packets. This is the most widely used method.
- SIP INFO: key presses travel in the SIP signaling instead of the audio.
When callers press keys and the IVR does not react, the two ends usually expect different DTMF methods, so it is one of the first settings to check on a SIP trunk.
In OVOKY, callers choose IVR options with their keypad, and agents have a keypad in the browser agent workspace to send tones during a call, for example to get through another company's menu.