discrepancy in ringback doc/code

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Peter T. Breuer writes:
> You know when the other end has hung up after only 6s without a
> ring after at least the first ring and anyway you could just tell mgetty
> to go into ringback mode when it receives say 3 rings (RINGS = 3) 
> and then come out of ringback mode when it receives a next ring within
> TIMEIN seconds and then stop waiting after TIMEOUT seconds have elapsed
> without a ring. This requires TIMEIN and TIMEOUT configuration variables.
> Set TIMEIN=6s (or 30s?) by default and you have backward compatibility.

There is one problem - in the UK, we have a 'Ringback' service which works
as follows:

Person A calls person B, person B answers.
Person C calls person B, and they receive the engaged tone. They can then
 hit '5' to select ringback.

When person B finishes talking to A, person B's phone rings.

Ok, now take this situation: person B does not answer the phone, but the
answering machine does after 4 rings. He leaves no message, so the
answering machine disconnects after 4 seconds of OGM and 4 seconds of
IGM. Meanwhile, person C has called and hit '5' as above.

What happens? Person A's phone rings within possibly as little as 10
seconds of the first call. If it happens to have an mgetty with ringback
on the line, it'll incorrectly answer the second call.

I don't think that the current mgetty behaviour is correct for this
situation, and some improvement along the lines above would be most
welcome to cover this situation if you wish to use this facility.

However, I don't agree that you should be able to set it too low -
maybe impose a lower limit.

Also, it isn't a good idea to specify a set number of rings - if you call
though a PABX, then you simply can't tell how many rings the remote end
will ring, since the clear-down time of the PABX is indeterminent.

What I suggest is to keep the current behaviour and to allow the
17 seconds to be a configuration option. This will then allow you to
specify all timeouts for ringback, and will minimise the changes to
mgetty.
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