The solution quoted in Outgoing call, 9 second delay - #10 by RichieH doesnât really make sense. It speeds up the sending of the number, but the value suggested is the absolute minimum allowed by SIN 351, and the change from 60 to 40 would only make a 1/5th of a second difference in the time to set up a call to a 10 digit number.
Best practice would, I think be to use something more like 150% of the minimum, so the 60% he claims doesnât work would seem to me to the reasonable value to use. A human dialling would, almost certainly significantly exceed this. I can successfully dial 1471 with digit and pause lengths over 500ms each, so long ones are not a problem, at least for my exchange, which was System Y (Ericsson AXE), but might have changed. (Simple phone, directly to domestic line.)
My feeling is that this is to do with how the HT813 handles answer detection, which is always a problem when using analogue lines, particularly domestic ones or one man type business ones. Unfortunately, the administration guide does not explain how answer detection is handled.
(If you used Asterisk, directly with analogue hardware, I think your choices would be answer supervision by a line reversal, or using dialplan logic on the audio (e.g. silence detection). I suspect, in your case, line reversal is not a service that will be provided, and it is not clear that that the HT813 would support it. It looks as though it doesnât offer an option of answering the SIP side immediately after sending the digits, which is what Asterisk DAHDI would do, with line reversal detection off.)
Are you picking up the called phone very quickly. Itâs possible the HT813 looks for ring back tone, followed by silence and signals SIP answer when the next round of ringing doesnât start when expected. Maybe some echo of the last digit is being recognized as ring back tone, in the other 9 second delay instance that you reference, and the 9 seconds is the fallback timeout for never receiving ring back tone.
The normal advice when answer or disconnect supervision is important, is not to use an analogue interface.