I gave my amiga 1000 keyboard cable to a friend so she could complete her Amiga 1000 setup. I then ordered some replacement RJ12 cables (4 wires!) to get mine working.
But they didn't.
Let's talk about why.
Firstly - yes, the cable is a RJ12 4P4C rollover cable. Ie, if you hold both connectors up next to each other and aligned the same way, the left hand pins are numbered "1-2-3-4" and the right hand connector is "4-3-2-1". Don't get this backwards or you'll end up reversing the power to the keyboard and damage stuff. It seems most phone cables are 4-wire RJ-12 and rollover pinout, but it's good to double check.
This is different to the early Macintosh keyboard - the RJ12 cable there is straight through. "1-2-3-4" goes to "1-2-3-4".
But it didn't work. I pulled apart the keyboard and started debugging it ... way too hard. The TL;DR is this. When I powered the keyboard from a 5v dedicated supply it was pulling 5v at around 125mA.
The cable I was using, straight from the bag:
The pinout is fine, but each leg has a 40 ohm resistance. There's no way to get 125mA out of 5v at 80 ohm resistance (+5v and GND, 40 ohms each.) The voltage on the keyboard side was closer to 2v.
The one I build/crimped until it worked:
18 ohms now, and can supply ~ 250mA. It was happy with this.
So if you're looking to replace a keyboard cable with an RJ11/RJ12 from Amazon or some other store, double check the pinout, double check that there's 4 wires in the cable, and double-check the series resistance!