Yes and no.
With Network Services three way calling, it is possible to introduce a third caller. There is supposed to be a facility (it was mentioned in one of the BT network guides) to introduce 8 callers. If everyone has three way calling, you can go on adding people to the call; this can even be done with several people in the US and several in the UK all linked through 3-way. The only problem is the voltage on the line keeps droping with every line added so eventually it gets very hard to hear everyone: it's ok for 2-6 people, for anymore it is better to set up a conference call.
However, if you have a PABX, you may be able to do this anyway using the conference call facility. You can also use BT's conference call service for three to sixty participants- call 0800 282429 for information.
[For a more detailed discussion see 'Caller Display and Call Return' by William Dangerfield, Simon Garrett and Melv Bond in British Telecommuncations Engineering; Volume 12 part 3 (October 1993). Also See Supplies Information Note (SIN) 227, available on 0800 318601. BABT have issued a specification, BABT/SITS/94/53 to replace the draft specifications BABT/TC/128 and BABT/TC/131. This specification is identical to the 131 draft and is shorter and easier to meet than the 128 draft. A copy can be obtained from BABT on (01932) 222289.]
The system described here is that developed by BT for use on the UK PSTN. It is based on the Bellcore 'CLASS' standard. This has the benefit of allowing CPE manuafacturers to base their UK models on those developed for the North American Market.
Most of BT's customers are connected to System X, AXE 10 or TXE4 exchanges and these exchanges are digitally interlinked using CCITT [now ITU] C7 signalling. C7 provides a way of passing the number of the calling number to the distant exchange (this information is used during call tracing).
When a call is made to a customer with Caller Display the distant exchange requests the number of the caller originating the call from the exchange at the other end of the C7 link.
If the call is not routed totally over C7 links (e.g. the caller is on an old analogue exchange), or the caller is on an interconnected network for which no agreement for the exchange of additional call information is in force, the number will not be complete. In this case customer with Caller Display will get a 'Number Incomplete Message'
If on the other hand the caller has deliberately withheld the number, by use of the 141 prefix the Caller Display Customer will get a 'Number Withheld' Message.
If the number is complete, and not withheld by the caller the number is routed on to the Caller Display customer over the local access network. For this purpose a V.23 sender has to be installed at every exchange concentrator.
When a line is about to receive a call the polarity of the line is reversed prior to the ringing current being applied. If the customer has Caller Display additional messages are interspersed between the polarity reversal and the application of the ringing current.
First a tone alert signal is sent and then an alternating series of '0's and '1's lasting 250ms is sent by the V.23 sender to assist the CPE in detecting the imminent arrival of the Caller Display message.
The Caller Display message takes roughly 0.75 seconds to send, after which the normal ringing current is applied to the line.
There are now at least three two line systems which do not require either