[russell_w_b]

Have you undertaken fundamental continuity checks on the plug and lead?

Have you tried a similar telephone on the socket you are trying your 706 on?

Don't get hung up on core colours or size of conductors. It's a telephone, not the National Grid. Connect your standard BT plug to your telephone as follows:

Looking at your BT plug with the pins uppermost and away from you... )Note: pins 1 and 6 aren't used).

Pin 2 connects to terminals 16, 17, 18, 19.
Pin 5 connects to terminal 8.
Pin 4 connects to terminals 5 and 6.
Remove the link between terminals 4 and 5 and replace it with a 3K or 3k3 resistor. Note that this isn't essential if it's the only instrument on the line! But 4 and 5 need to be connected either by link or by resistor.
Remove links between terminals 6 and 7 or 8 and 9, or both, if you wish.

Note also that pin 2 of the plug mates with terminal 5 of the socket rear connection; pin 4 with socket terminal 3, and pin 5 with socket terminal 2. If your socket is wired correctly you needn't worry about this. Just get your plug connections correct WRT the telephone terminals.