The MF tones in R2 are compelled, meaning, there is a sequence per
tone, imagine the first DNIS transmitted in a connection is 7, the tone
sequence is:
1. Forward side tx tone 7.
2. Back side detects tone 7 and then sends a tone too. That backward tone serves
2 purposes: both acknowledges the reception of tone 7 and is also a request
for either the next DNIS, or next ANI or something else.
3. Forward side detects the tone request from the backward side and mutes its
own tone.
4. Back side detects the silent condition of the forward side and cancels its
own tone.
5. Forward side detects silent condition from the backward side and proceeds to
send the next tone depending on the request sent by backward side in step 2.
This is called the compelled sequence. The MF Back Timeout controls how much
time will the backward side wait for its request to be fulfilled. This
timeout is rather important in some countries because some variants do not
support a signal for "end of DNIS", and they rely on this timeout. If the
backward side does not see a response in x amount of milliseconds to its
"get next DNIS" request, it will assume the forward side ran out of digits and
it will resume the compelled cycle by issuing another request (like get next
ANI or get category).