I am in the UK.
>
>I have a Virgin Media/NTL landline and am considering installing a BT
>landline too. (I need Caller ID but VM/NTL don't have it in my area).
>
>If I have answered a call on VM/NTL and also on BT then how can I join the
>two lines to have a 3-way conversation?
>
>Is there a circuit I can build which will join the two lines?
Look up circuits called "4 wire terminating set", or "4 wire hybrid".
And perhaps a "4-way 4-wire bridge".
Understanding those will make it obvious how to properly bridge two telephone lines. Just be aware that it is not as simple it sounds...
(1) Perhaps picking up the signals by a transformer across each of the two
>line cords?
Close.
(2) Perhaps across each curly lead which goes from handset to phone?
Don't even think about that... ;-)
(3) Would I get better results "audio-coupling" the handsets. Holding one
>earpiece close to the other mouthpiece and vice versa?
Awful idea.
Here are the problems...
Each telephone line is designed to be terminated in about 600 Ohms impedance, and deliver about a -9 dBm signal to the telephone set. It expects to get about a
0 dBm signal from the telephone set.
Okay, if you simply place two telephone sets across the same line, or if you have one telephone set and two lines, you split the power coming from any of the three sources (line, set 1 or set 2) two ways into the other two devices. That drops the signal level by 3 dB for both of those devices. That might work if the connection is good and the speaker is loud and clear. If the connection is bad or your speaker is quiet, it's a bad deal. (In the case of two lines, you have to deal with the DC loop currents on each line separately from the AC voice signals.)
The trick is to split the signal from a line and then amplify it 3 db for each split. The problem is that you cannot put an amplifier directly into a telephone line because there is a signal going in both directions and amplifiers are one way devices. You need a hybred circuit to split off the transmit and receive directions so that an amplifier can be put into each direction.
+--------+ TX 1-way|\\ +------+ 2-way| |->-------| >-----| | Line 1 >=====| Hybrid | |/ /| | | | |--------| >-----| | Line 2 >=====| Hybrid | |/ /| | | | |--------| >-----| | telset 1 >=====| Hybrid | |/ /| | | | |--------| >-----| | telset 2 >=====| Hybrid | |/ /| | | | |-