How does a UK analogue telephone detect that the caller has hung up (disconnected)?

Question

Scenario This is a question regarding the design of UK analogue telecommunications equipment and the consequent design impact that is necessary for a router duplicating those signals. It is asked so that I can liaise with the designers of the router so that they can alter the design of the router to resolve the problem stated. This question is not about how to use a router or other equipment.

Here in the UK I have an analogue answerphone that worked perfectly when it was plugged directly into the old, analogue master socket and recorded messages finished immediately the caller hung up. Since March I now have a router with VOIP capability (Fritz!box 7530) that generates/receives UK analogue telephone signals at a FON socket, allowing old analogue phones and equipment to work using my new VOIP Internet connection.

The problem

Calls work fine with the analogue phones and an answer phone connected to the FON socket will record messages left by callers. However when the caller hangs up after leaving a message I get a series of beeps (number unobtainable tone?) recorded after the message for nearly a minute. I think this is until something times out and the resistance of the line goes up to almost open circuit, indicating there is no call in progress.

The issue seems to be that the router is not designed to generate the correct tone to indicate to the answerphone when the caller has hung up or, if it is, then the frequency or cadence of that tone is not quite right.

Question

Exactly what 'signal' does BT require to be designed into equipment and sent down the line when the calling party hangs up a UK landline phone (and which was previously being detected by my answerphone with no problems)?

If it is one of the usual tones such as number unobtainable or engaged then I'd like to know exactly what frequencies the design calls for and what cadence (mark/space between pulses) is used?

Places I've already looked

Lots of web sites and several questions on Stack Exchange. However most of these either refer to American systems or they don't say and are presumably American. I need the UK protocol.

Further information

I've been in contact with the German router manufacturer who are willing to alter the design of the router if they know exactly what to alter it to.

Answer 1

I need the UK protocol

Wiki - Disconnect tone gives you details for the UK: -

A disconnect tone in telephony is a tone provided to the remaining party to a call after the remote party hangs up. Typically, the disconnect tone is a few cycles of the reorder or busy tone (e.g. in US), or between five and fifteen seconds of the Number Unobtainable tone (e.g. in UK).

So maybe your new system doesn't produce a number unobtainable tone that sufficiently matches what is hinted at above. The number unobtainable tone in the UK is a continuous tone of 400 Hz.

See ITU Operational Bulletin No. 781 - 1.II.2003 for a list of all number unobtainable tones used throughout the world.

Answer 2

Well, hm, analog telepone specifications are often a bit messy, as historically, network operators often were national, and they had the power to develop and type-approve on their own, so the way standardization happened was often following specific regional experiences with "yeah that's how we used to do it".

The relevant UK standard is British Standard BS 6305, which specifies how your British phone ought to work, electrically.

However, and that's what I mean with messy, the way phone and network interact used to be standardized by British Telecom PLC themselves, in SIN 351. I think you're looking for this, Section 7 "Call Clearing", specifically "7.2 Network Initiated Clearing":

The BT network interface will provide a sequence of clearing signals at the NTP as a result of terminals ending a call or when terminals fail to present valid digits during call set-up. This will consist in any order of:

a) an ‘end-of-call’ signal of between 90 ms and 130 ms and/or;
b) number unobtainable tone lasting between 3 s and 20 s and/or;
(Number unobtainable tone is described in SIN 350 [3] Network Tones and Announcements)
c) silence lasting between 0 s and 30 s;

and will end with the Parked State (see section 7.3 Parked State).

Note 1: The ‘end-of-call’ signal is sometimes known as the “K-break” signal. It offers a positive way for automatic terminal equipment to determine when either a calling terminal or the BT network interface has resumed the off-line condition. The signal consists of a disconnection or a reduction in the loop current to below 1 mA for the time period stated.

Note 2: There are certain interfaces supported by non-copper access systems that cannot provide the ‘end-of-call’ signal.

My bet is your answerphone is expecting a disconnect, so option a) from that list, but it gets b), and possibly in a different tonality than a British phone expects.

I know that at least in the early 1990s in Germany, a phone where the other end hung up would stay "on line" for more than a minute before the network disconnected the loop. So, that might be the case here.

Answer 3

This is called "calling party control", "far end disconnect", or in BT terminology "disconnect clear time". It consists of making the line open circuit (0 volts tip-ring) for a duration somewhere between 50 and 1000 milliseconds after the remote party hangs up. The duration varies quite a bit around the world, and the most likely problem is that the signal your ATA generates is shorter than what your answering machine is willing to recognize. In that case they simply need to make it longer (e.g. 500ms).

In some times and places, the disconnect signal was a polarity reversal, instead of open-circuit. This isn't used in the UK. A less likely cause is that the ATA is configured for the wrong standard entirely, or isn't signalling far-end disconnect at all. In that case it needs to be configured for the correct standard.