running: Add note about DTMF support

osmo-sip-connector does not yet support full DTMF support. The current
implementation only supports DTMF tones to be send from MNCC to SIP,
but not in the opposite direction.

Change-Id: I578e50b0a42d88b05cf6da80443b71494b5eb26f
Related: OS#2777

doc/manuals/chapters/running.adoc

@@ -56,3 +56,29 @@ mncc
  socket-path /tmp/msc2_mncc
 ----
 
+=== DTMF signaling
+
+In VoIP based telephony networks DTMF (Dual-tone multi-frequency signaling) can
+be signaled through multiple methods. Common methods are in-band, RFC2833 and
+sip-info messages.
+
+Osmo-sip-connector is using sip-info messages to signal DTMF tones. When a DTMF
+tone is signaled at the MNCC socket interface, osmo-sip-connector will generate
+a matching sip-info message to forward the DTMF signal to the PBX. Depending on
+the PBX software reconfiguring the DTMF signaling method to sip-info may be
+necessary.
+
+While sending DTMF tones through the MNCC interface to a SIP leg, the current
+implementation of osmo-sip-connector does not support sending DTMF tones in the
+opposite direction. Any attempts to send DTMF tones to an MNCC leg will be
+confirmed with a status 405 "Method not allowed".
+
+The reason for this limitation is that in mobile networks, depending on the
+signaling direction, the signaling of DTMF tones is implemented differently.
+A mobile originated DTMF tone is signaled through out of band messages,
+which arrive at osmo-sip-connector on as MNCC DTMF START/STOP messages. Those
+messages can be directly translated to sip-info messages. However, in the
+other direction (mobile terminated), an in-band signaling method is used. This
+means that osmo-sip-connector would have to translate an incoming DTMF sip-info
+message into an audio sample that then would have to be injected into the
+voice stream. Currently this scheme is not implemented in osmo-sip-connector.