Welcome, Ian, to the OEM forum.
Was this telephone cable or alarm cable? I ask because 4-core telephone cable doesn’t usually exist in long pieces, it’s actually 2-pair cable. This means, two pairs of wires individually twisted. Do you really have Tx and Rx terminals, or are they labelled A and B or + and - ?
You should be able to extend the bus to 15 m. As far as I’m aware, you’ll have a “half-duplex” bus which uses a balanced two-wire connection plus a reference ground. If this is the case, and you have a standard telephone cable, I’d use blue and blue/white connected to A & B legs, and the other orange pair together as the ground. If you have A & B legs swapped, this is likely to cause problems. (And if you think they are correct, it might be worth swapping one end - I think I read a long time ago about someone who had found a device which was labelled incorrectly.)
This should be worth reading:
Note particularly the bits about termination resistors. No termination or double termination will be equally bad.