worked out why it won’t dial out for a minute after previous outbound call (no dial tone or battery on the analogue circuit, as if the line is disconnected); but accepts inbound calls during this period (sending ringing voltage to the “telephone line”). I thought this was a bug/defect or some limitation of the 14 year old technology) but it is a guard interval deliberately added to prevent glare.
Glare is a problem with analogue telephone lines connected to automatic equipment, whereby a call coming in is wrongly answered by one being sent out, very confusing to both parties involved! Today it is less of an issue; anyone who has set up a small telephone system has most likely got it connected to VOIP trunks as well as the analogue line and the order in which these are selected can be set to use the ones least used for incoming calls first. in 2001 mobile calls weren’t always as cheap, PAYG customers didn’t get free minutes, contract ones got less and and having a telephone system in a small business wasn’t cheap either (hence why the device accepts loop disconnect dialling (used on the old dial phone).
A lot of equipment from the 1980s and even late 1970s (the PTT may have installed) was still in service. If the equipment glared a GSM circuit and folk got charged for the calls or it ate into the free minutes everyone would be pissed off (including the telephone companies, who would get blamed for it; in some countries the Communications Ministry could order the whole lot to be disconnected).
Users connected to a telephone system (even if they are not technical) know the number they dial has to be stored, older people remember dial phones and are thus used to waiting for an outgoing call to complete, but expect inbound calls not to be delayed other the minimum time required to confirm the line is ringing and get caller ID if present and route the call to one or more extensions as desired.
So after each outgoing call the guard interval starts, the “battery” (-48V DC) is removed from the telephone line circuit for 60 seconds (both humans and machines recognise this as a “dead” line and select another telephone line (if one was available). A human calling via an automatic phone system would either hear an error tone or silence long enough for them to realise there was nothing at the other end; and do something else like have a cup of tea or relight their pipe, or go outside and feed goose ducks etc.. (these adapters were also widely sold to Asia to extend the range of telephone systems when the fixed phone company had run out of capacity). At least no one will not be charged for dialling into a disconnected line (which was a common occurence anyway in many countries).
I checked the ETSI document that the interface complies with (997 pages, at least I only need to look at some sections!) Even much old equipment from mainland Europe is smart enough not to dial into a line with no battery (its only UK and some USA kit which does this) – DK is particularly fussy about telephones working as they should even in adverse line conditions), this document warn that battery may be disconnected at various points before and after a call (not during one unless some idiot has cut the line!) so connected equipment should expect this anyway. Incoming GSM calls will always ring the line (cancelling the guard timer) so these are not missed.
60 seconds is equivalent to dialling “0” 20 times on a loop disconnect circuit (it is 10 pulses so takes the longest time to dial). That would be (00 + 18 more) as a worst case scenario and an impossible number as 9 and 0 and 1 are not usually used to start telephone numbers; the first two are used for trunk/operator access and emergency numbers, and 1 is easily accidentally “dialled” by faulty lines or equipment with intermittent disconnections.
So this is not a fault a smart move; which guarantees this GSM terminal works in all of Europe no matter what ancient stuff is connected to it
I had simply forgotten that people were more used to waiting for phone calls to dial and complete! I am unsure if it is possible (or permitted) to set the device to tone dialling only as the config software is abandoned; not not a big problem as I have a second SIM on the same network in another mobile phone anyway, VOIP trunks, as well as the analogue line and other backup kit even if the North Sea floods half the town, the whole of BT is half drowned and the goose ducks are on my lawn; but having not come across a guard time this long before thought it worth noting down somewhere, as they exist on other telecoms equipment and younger people may have to set this up and be confused by this issue…
