PJSUA2 and PJSUA-LIB support sending DTMF digits as inband tone, RTP events (RFC 2833), or SIP INFO. On the receiving side, the libraries support reporting DTMF digits sent as RTP events (RFC 2833) and SIP INFO. Note that detection of inband DTMF tone is currently not implemented.

PJSUA-LIB (and inherently PJSUA2) will only send RFC 2833 DTMF if remote party has indicated its capability to accept RFC 2833 events in its SDP. This is done by putting this line in the SDP:

a=rtpmap:101 telephone-event/8000

Without receiving this capability indication, PJSIP will refuse to send RFC 2833 event, and pj::Call::sendDtmf(), pjsua_call_send_dtmf(), pjsua_call_dial_dtmf() or pjmedia_session_dial_dtmf() or pjmedia_stream_dial_dtmf() will return error code PJMEDIA_RTP_EREMNORFC2833.

Communicating DTMF using SIP INFO is considered as proprietary and have not been standardized (see RFC 6086#section-2). This mechanism is also very expensive in terms of network resources used. Use this mechanism with care. Please see ticket #2036 for more information.


Send DTMF with pj::Call::sendDtmf() method, specifying the method in pj::CallSendDtmfParam::method field.

Incoming DTMF will be reported in pj::Call::onDtmfDigit() callback.


Send DTMF with pjsua_call_send_dtmf() function, specifying the method in pjsua_call_send_dtmf_param::method field.

Incoming DTMF digits will be reported in pjsua_callback::on_dtmf_digit2 callback.

Sending inband DTMF tones

Below are steps to send inband DTMF tones:

  1. Once the call is established, create an instance of Multi-frequency/DTMF Tone Generator.

  2. Register this tone generator to pjsua’s conference bridge with pjsua_conf_add_port().

  3. Connect the tone generator to the call, with pjsua_conf_connect().

  4. Now instruct the tone generator to play some DTMF digits with pjmedia_tonegen_play_digits(). The digits then will be streamed to the call, and remote endpoint should receive the DTMF tone inband.

Below is the snippet to do it:

struct my_call_data
     pj_pool_t          *pool;
     pjmedia_port       *tonegen;
     pjsua_conf_port_id  toneslot;

struct my_call_data *call_init_tonegen(pjsua_call_id call_id)
     pj_pool_t *pool;
     struct my_call_data *cd;
     pjsua_call_info ci;

     pool = pjsua_pool_create("mycall", 512, 512);
     cd = PJ_POOL_ZALLOC_T(pool, struct my_call_data);
     cd->pool = pool;

     pjmedia_tonegen_create(cd->pool, 8000, 1, 160, 16, 0, &cd->tonegen);
     pjsua_conf_add_port(cd->pool, cd->tonegen, &cd->toneslot);

     pjsua_call_get_info(call_id, &ci);
     pjsua_conf_connect(cd->toneslot, ci.conf_slot);

     pjsua_call_set_user_data(call_id, (void*) cd);

     return cd;

void call_play_digit(pjsua_call_id call_id, const char *digits)
     pjmedia_tone_digit d[16];
     unsigned i, count = strlen(digits);
     struct my_call_data *cd;

     cd = (struct my_call_data*) pjsua_call_get_user_data(call_id);
     if (!cd)
         cd = call_init_tonegen(call_id);

     if (count > PJ_ARRAY_SIZE(d))
         count = PJ_ARRAY_SIZE(d);

     pj_bzero(d, sizeof(d));
     for (i=0; i<count; ++i) {
        d[i].digit = digits[i];
        d[i].on_msec = 100;
        d[i].off_msec = 200;
        d[i].volume = 0;

     pjmedia_tonegen_play_digits(cd->tonegen, count, d, 0);

void call_deinit_tonegen(pjsua_call_id call_id)
     struct my_call_data *cd;

     cd = (struct my_call_data*) pjsua_call_get_user_data(call_id);
     if (!cd)


     pjsua_call_set_user_data(call_id, NULL);

The resources that were allocated above must be released once the call is disconnected, by implementing this in pjsua_callback::on_call_state callback:

static void on_call_state(pjsua_call_id call_id, pjsip_event *e)
     pjsua_call_info call_info;

     pjsua_call_get_info(call_id, &call_info);

     if (call_info.state == PJSIP_INV_STATE_DISCONNECTED) {

With the above snippet, call call_play_digit() send inband DTMF digit to remote party.

Implementing inband DTMF detector

Currently PJMEDIA lacks built-in tone detection routine. If tone detection routine is available, it should be straightforward to integrate it to the framework:

  1. Wrap the routine as PJMEDIA Port so that it can be plugged to the media framework. The implementation would be similar to WAV recorder media port (pjmedia/src/pjmedia/wav_writer.c), but instead of writing to WAV file, it would monitor the audio signal for tone and call some callback when a tone is detected.

  2. Once the tone detector media port is implementation, add this media port to the conference bridge with pjsua_conf_add_port(), and connect the audio source to your tone detector with pjsua_conf_connect().