Which wires are the send (transmit) pair and which are the receive pair?

The RJ11 plug is a 6 position plug with the center two wires (pins 3 and 4, which are typically red and green) as the transmit pair. The pins on each side of transmit pair (pins 2 and 5, which are typically black and yellow) are the receive pair.

1 N/C
2 Receive 1
3 Transmit 1
4 Transmit 2
5 Receive 2
6 N/C