This has also been known to work on the ET-237.
First, download the software.
The 1341 serial interface operates at TTL (not RS-232) levels, so you must use a converter (ie) max203). The pins are numbered from left to right with the phone face up.
Using the MAX203 converter and a DB-25 for the computer's serial port:
Computer DB25 Phone 2----------MAX-203------------11 3----------MAX-203------------12 7-------------GND--------------8 | 8--- 5--- | 6---