Revolabs FLX2 VoIP Advanced - Manual (Page 65)

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useful when messages might be too large to be carried on the
available transport, for example when exceeding UDP’s Maximum
Transmission Unit (MTU). Enable this option to encode SIP
headers in their short forms to reduce size. By default, the option
is not enabled and SIP headers in outgoing messages will be
encoded in their full names. (See SIP protocol standard, IETF
RFC 3261.)
DTMF Signaling
Method Select the signaling method for transmitting DTFM tones, either
via RTP (RFC2833) or SIP INFO messages. The default is RTP.
Media on-hold
method The Media on hold method setting allows switching the Media on
Hold behavior between the different RFC definitions.
“M line only (RFC3264)” implements the functionality according to
RFC3264. In the INVITE message SDP body, the attribute
“a=sendonly” is set to a designated media stream to put media on-
hold.
“M and C line (RFC2543)” implements the functionality according
to RFC2543. In the INVITE message SDP body, the connection
line ip is set to “0.0.0.0” (e.g. “c= IN IP4 0.0.0.0”), and the attribute
“a=inactive” is added.