This method separates DTMF digits from the voice stream and sends them as ASCII characters in H.245 user input indication messages through the H.245 signaling channel instead of the RTP channel. DTMF-relay works around this shortcoming by sending the tones out-of-band, or seperate from the encoded voice stream.įor IP phones on H.323 networks, DTMF is relayed using the H.245 alphanumeric method, which is defined by the ITU H.245 standard. The trouble with sending DTMF in-band is that the codecs used may distort the DTMF tones and subsequently make them unrecognizable to the receiving system or device.
These are used to navigate voicemail prompts, auto-attendants, IVRs, and the like after a call has already been established. DTMF (dual tone multi-frequency) is in simpler terms, the tones that are generated when you press a digit on the keypad.