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).